Maybe we need the ability to build some sort of room for children -- a creche or schoolroom or something like that. Children would spend most of their time there, and would be taught skills by dwarves who were set to the appropriate job. Perhaps they could pick up some skills by watching their parents at work if they're not in the creche, but overall, dwarven children are rarely (if ever) seen in fantasy, so it makes sense for them to be kept safely in a hidden underground creche. An adult dwarf assigned as a teacher would also have the job of keeping them there; while a child is in the care of the teacher, parents don't worry about it.
Regarding the usefulness of children, once they're old enough to stop crawling, I think they could help out more, too... Dwarves tend to value industry, and I doubt they have strong child-labor laws. A child could help their parents work on many different things... There could be a 'child bonus' to the speed of many jobs (like metalworking or planting) if a parent's child is nearby, to represent the child handing them things and so on.
They could handle some hauling, even, at least for small objects (and dwarf children are strong anyway!) They could occasionally produce small crafts on their own initiative, especially if their parents are craftdwarves. They could play with wooden toy weapons, and even use them for self-defense in an emergency.