"The baby's pillow"
Translation:Mto wa mtoto
Mtoto is a perfectly acceptable translation of baby. A mtoto is any pre-adult person. If you need to specify, you can say mtoto mchanga or mtoto mdogo (which could simply mean a small child), but you don't have to.
Words don't generally have exact equivalents from one language to another.
Yeah, it should accept more specific phrases too, but mtoto is not wrong.