My son is 2 1/2 and had no trouble ripping these door knob covers right off! Pretty sure a monkey could defeat them. I will say that the actual mechanism for squeezing and turning the knob is good; he can't do that. What he can do, however, is pull the two halves of plastic apart and take the knob cover clean off. But, since these were so inexpensive, I guess I shouldn't have expected too much. A little home engineering and I have gotten them to work for us: duct tape. Taping the cover around the knob keeps it from being popped apart and removed. Now it is securely on the knob, spins when he tries to turn it, and has the little tabs that I can squeeze through the tape and open the door.In short, if you don't mind having to alter the product a little, it can work. But, if you're planning to put them on every door knob in your house, I wouldn't recommend them.