I have heard it said that this is related to breast feeding, but since the babies I have smelled were both breast fed and had sweet breath, I can't verify or reject this notion. However, babies and very small children can have bad breath. The other kinds are similar to the smells adults have. Very young children can have smell from the back of their tongues. This odor can be removed by cleaning the tongue, although this is more difficult to perform with youngsters.
Young children can also have odor coming from their teeth.
In most cases, proper brushing can take care of this. In some cases, in preteens and teenagers, cleaning between adjacent teeth may be extremely important to minimize bad breath.