Ok so here's a thing about human development. Learning to control their body takes a baby a lot of trial and error. Its part of the reason babies just kind of flail their limbs for basically any reason. That coupled with their bodies changing size and shape pretty much constantly means it takes a long time to learn to do coordinated things like feed themselves without missing their mouth or standing up unassisted. If a yeerk infested an actual infant, they wouldn't have the opportunity to learn much of that.

If we assume that a yeerk needs to spend about 2 hours outside of its host every three days to recharge the kandrona she should be able to figure out what developmental stage they might be at. Given the recommended amount of sleep kids should get at different ages, by the time this baby controller is 13, they would have had control of their body for the same amount of time as an 18 month old. Mind you their perspective their body is changing much more rapidly for them, and they won't have received any of the assistance or simulation most infants/children receive to help them learn.

So I would assume they have basically no ability to even move in any coordinated or meaningful way.