I suspect that in the Walking Dead universe there are the expected number of zombie children roaming about, we just don't see them in the TV show
(I haven't read the comics so I can't comment on those) for a couple of out-of-universe reasons.
I imagine working with adult actors is easier than working with child actors.
No need for parental consent, less restrictions around working hours, etc.
If you need a whole host of extras to act as zombies, adults are just less hassle.
The audience is more likely to be comfortable seeing an adult zombie being killed than a child zombie.
While they're reasonably dehumanized, the show having zombie children walking around and being killed by the survivors in every episode is likely to put some viewers off.
Plus, honestly, who wants to see at least one of the group agonize over killing a child in every single episode?
scifi.stackexchange.com/users/5833/Anthony Grist
Getting your grub on, as a zombie, is a competitive sport.
And like most competitive sports, the strongest and fastest persist and continue to feed.
The zombies in the Walking Dead have not had enough explanation given to their movement, their continued existence or how they retard the destruction of their now barely-living flesh
but if we were to consider them viable, if undead, organisms, the reasons we don't see more children becomes more apparent.
If the lives of the undead are sustained through the consumption of flesh, child zombies are at a disadvantage.
Smaller, slower, and weaker simply by being children, they get less flesh to eat, have to work harder to get it and will be pushed aside by larger adult zombies with greater strength/mass (such as it is) to work with.
With that in mind, they will eat less often and thus have less fuel to work with.
Child zombies have less mass so their rate of decay may actually be faster because there is less of them TO decay.
Given they weigh half of what the average adult does, I would expect them to decay faster, lose flesh faster and during cold weather, freeze faster and more solidly than their larger counterparts.
If brain destruction is part of the process of rendering a zombie dead, a child brain should freeze faster than an adult one because there is less zombie flesh and bone to get in the way.
Humanitarian (again, a relative term)
parenting: If people were able to suss out that dying meant coming back as a ravaging, flesh-crazed attacker of the living, parents might opt to put their sick children down upon death (chopping off heads, impaling brains, or shooting them in the head) to prevent them from rising up.
This act of kindness/self-preservation, may be the simplest and most effective reasoning for the lack of child zombies from an in-world prospective.
Other answers have tapped the production rationales for not seeing as many child zombies.
Workload, scheduling, parents, school schedules and the like.
scifi.stackexchange.com/users/2765/Thaddeus Howze
Single-serving children don't leave enough corpse to become zombies.
In the Walking Dead universe the primary way one becomes a zombie is to be bitten by a zombie and expire from the ensuing infection.
The thing is, it was the intent of the zombie that inflicted the bite to eat said individual.
One must possess the wherewithal to end one's encounter with a zombie in few enough pieces to become a viable zombie oneself.
Defenseless, bite-sized children are unlikely to escape said encounter alive and unlikely to be enough of a meal to satiate a zombie before becoming an unreanimateable collection of bones.
scifi.stackexchange.com/users/14435/nwellcome
So far I think they've had 1 zombie kid per season...
Season 1- the little girl Rick killed in the 1st scene, Season 2- Sophia and Season 3- Penny.
Season 4, they are pretty much implying "all bets are off" & there's a lot more people at the prison now
that came from Woodbury, mainly Women & kids, because The Governor took all the others with him to attack the prison & ended up killing the ones who ran, so with the population at the prison
, you might see more of it this season...
scifi.stackexchange.com/users/17916/Kim
I think it is mainly an production decision.
People (especially parents) are likely to respond negatively at seeing children's corpses walking around eating people before being shot in the head.
As an example I can cite my wife.
She was never a big WD fan
but once she saw the Governer's daughter she was unable to watch the show again and does not even like me talking about it
(we have two young children ourselves) -
luckily she missed the one about Sophie
and I was able to lie to her about what happened.
scifi.stackexchange.com/users/6581/Stefan
When zombies bite children, they do not just bite, they eat it all!
scifi.stackexchange.com/users/8218/scythargon
It's probably mostly a production thing.
As a mother myself, it tore me apart to see what happened to Sophia, whom we were given time to bond with as a character.
It shocked me to see what Rick had to do to the little girl in season 1.
Over time, maybe, I've become a bit desensitized to the zombie slayings (Penny didn't bother me) but the scene of the empty carseat on the highway and bits of flesh scattered ripped through me.
Mothers, especially, would struggle to stay with the show if too many kids were eliminated, either by attack or shot after zombification.
Sometimes I have to pause and take a break from it as is; I can't imagine that I'd be able to keep watching if there were more kids in it.
scifi.stackexchange.com/users/19605/Amy
They are in good part waist high or shorter and just get overlooked.
Not only that but when a zombie starts eating kids they don't leave a lot to be turned as they can be eaten more quickly especially in a crowd.
scifi.stackexchange.com/users/20112/freakngeckos
In The Walking Dead, why aren't there more children zombies?
In The Walking Dead....why aren't there more children zombies?
scifi.stackexchange.com
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scifi.stackexchange.com/questions/35370
scifi.stackexchange.com/users/14430/Seneca
Although they are reasonably dehumanized, the show having zombie children walking around and being killed by the survivors in every episode is likely to put some viewers off.   Also, child zombies would decay or freeze faster due to their lower mass. The audience is not likely to respond well to more children as zombies due to the killing of zombie children that has occurred in the past. Also, from a production perspective, adults are easier to cast. It is also a consideration that children are eaten whole and decay faster than adults so we see fewer of them — they are also shorter and less visible than adults. 
The audience is not likely to respond well to more children as zombies due to the killing of zombie children that has occurred in the past. From a production perspective, adults are easier to cast. Children are eaten whole and decay faster than adults so we see fewer of them — they are also shorter and less visible than adults. 