OK, so I thought it was "watoto wadogo" which is what I learned I in the Swahili classes I took, but that wasn't accepted here. They gave the translation "watato wachanga" instead as well. I just checked the TUKI Kamusi (dictionary) of the Institute of Kiwahili studies at the University of Dar es Salaam, and they do indeed list "mtoto mchanga" for baby. So then I checked "changa" on the Swahili side, and it means "young, infant, immature," and mtoto mchanga as baby or infant...
watoto can be used for babies too as well as older kids. But if you wanted to be really specific about the fact that youre speaking about babies or infants you could say watoto wadogo or watoto wachanga. also who can be classified as a child is very relative, you can hear elders refer to you as mtoto even if youre in your 20s so theres that.