Generally helmets are recommended for carrier usage by manufacturers and governing bodies, although local cultural practices may vary depending on perceived risks.
Helmets are recommended because it is still possible to crash or even roll a trailer, just as it is possible to crash on a bicycle, even if you are riding slowly and in quiet areas.
While children should be wearing seat belts within the carrier/trailer, the structures would not provide adequate protection against head injuries, making helmets the last line of defense against a head injury in the case of a crash.
bicycles.stackexchange.com/users/4534/Rider_X
We opted for helmeting our kid whenever we use the trailer in bicycle mode (i.e. not as a pram).
This is why: Our neighbours managed to flip over their trailer once.
Kid got a nasty bump on the head, despite having been strapped in.
Establish from the start that bicycles and helmets go together, no discussion.
bicycles.stackexchange.com/users/38391/user38391
No, your kid does not need a helmet, if you have a proper trailer.
Proper trailers do have belts and rollover protection.
If you don't wear a helmet in your car, you don't need one in a trailer.
Smaller kids without proper neck muscles should be seated in a special seat (like the fitting car seats), which doesn't permit helmets.
Manufacturer advice is mixed: Croozer explicitly states the lack of need for helmet protection inside its trailers, Weber advises a helmet a protection against hitting the side or the other kid (Ritschies are two seated models).
As you state to live in southern Germany: if you travel to Austria, helmets are obligatory, even in trailers, until the age of 12.
Opinion : I find the focus on head injuries in biking excessive.
For trailers a very sturdy base
tub
(trailer is hit or trailer is damaged from below) and a secured connection (accidental separation at high speeds or on hard braking) is so much more important to avoid severe injuries.
Unfortunately the last systematic test by Stiftung Warentest is quite old now (2010).
bicycles.stackexchange.com/users/38392/Leonidas
I mainly go with @Leonidas: no helmet for children inside.
We have a quality trailer (from Chariot).
I like my children not to wear a helmet in the trailer because: A helmet puts their head in an uncomfortable position (pushes headsdown)
They may cause (smaller) injuries to their sibling next to them
In an accident, the inertia of the helmet may cause a whiplash injury.
Just to emphasize: This is highly controversial where I live (Switzerland).
I even do not agree with my wife on this.
As a fact Helmets are not required by law on a bicycle or in a trailer.
bicycles.stackexchange.com/users/38416/Chrglmgl
I too prefer a helmet, but many children that are well within the weight limit are too tall to fit inside with a helmet on (mine, for example, age 4).
Most trailers have a 5 point harness and a frame that acts as a roll cage, which combine to reduce the need for a helmet.
bicycles.stackexchange.com/users/7309/Chris H
I'm with the others in saying that a helmet is a good idea.
Not only in the event of a crash, but also just in getting in the habit of wearing a helmet while out on the bike.
For what it's worth, Ontario law requires that all riders under the age of 18 wear a helmet whether they be on a bike, in a carrier or towed in a trailer.
bicycles.stackexchange.com/users/163/Kibbee
If I had a child in a trailer, I would put a helmet on them.
Not as much needed as in a child seat, but still an added security factor.
The kid could bash their head on the struture in a sudden movement of the trailer.
bicycles.stackexchange.com/users/38357/Gabriel C.
Always get your child to wear a helmet, rolled the trailer Sunday with my 3-year old grandson inside and he had the worst head wound I have ever seen.
Horrific.
bicycles.stackexchange.com/users/45307/Paul Hallett
we have a 12 month old baby and a Burley Solo.
Where we live there is no mandatory rules on helmet wearing for any ages (which is a shame).
I am 100% on board with our baby wearing a helmet.
Although the construction is sturdy, and there is a quality belt system, I believe there are too many unknown scenarios to depend solely on the safety/contruction of the trailer and my ability to avoid an accident.
So the baby (as well as me) wears a helmet.
I agree the car analogy is erroneous, they are two completely different environments.
I agree that wearing a helmet from the get-go will hopefully instill this practice as a norm for the future.
Additionally the Burley Solo has extra space behind the head specifically for the helmet bulk.
bicycles.stackexchange.com/users/43658/user43658
I am thinking about buying a trailer for my 1yo daughter. They are quite common in my region (southern Germany) but I haven't seen a single kid with a helmet inside. I know that there is no law that regulates it. Does trailer's manual ask to wear a helmet? Or are there any official (not necessary German) recommendations about wearing it?
Does toddler need a helmet inside a bicycle trailer?
bicycles.stackexchange.com
<helmets><trailer>
bicycles.stackexchange.com/questions/55558
bicycles.stackexchange.com/users/38378/Raz
There ia a lack of consensus on whether children should wear helmets in trailers or not. Some people believe the safey harness and special seat for very young children combined with the structure of the trailer is safe enough. It would seem that regulations also differ from country to country: in Austria, for example, helmets are obligatory. However, the majority of people seem to think it is better for a child to wear a helmet inside a trailer for extra safety. Although, some trailers do not seem to have enough space for the child to sit comfortably with a helmet on. Finally, wearing a helmet from an early age may encourage the practice later on. Arguments against wearing a helmet in a trailer include the existing protection systems and the possibility that a helmet could put a child's head in an uncomfortable position and even cause injuries in a crash. Many trailers have a space that allows a child to be properly seated while using a helmet, so discomfort is not as likely.  However, these harnesses are not as secure as those on a car seat, and there is a greater chance of rolling or children hitting their head on the side or another child's head. In addition, helmets are a legal requirement in some countries. Using a helmet whenever the child is on a bike, child seat or trailer trains them to use a helmet in the future.
Arguments against wearing a helmet in a trailer include the existing protection systems and the possibility that a helmet could put a child's head in an uncomfortable position and even cause injuries in a crash. However, these harnesses are not as secure as those on a car seat, and there is a greater chance of rolling or children hitting their head on the side or another child's head. In addition, helmets are a legal requirement in some countries. Using a helmet whenever the child is on a bike, child seat or trailer trains them to use a helmet in the future.