I didn't read anything about this yet, if I am repeating please forgive me.
Once you get your hands on a lockpick and use it to get into a house, you can trade with the children that are in the house (if there are some cause they don't all have some).
If the children don't have what you are looking for or you want to trade with one more then once, leaving the house and entering it again will reset the inventory of the children inside.
This way you can stock up on the items you might need without running around town looking for children to trade with.
Some houses have up to three types of children in them.
Hope this helps