The second part of your question is worded confusingly, so I will try to answer it later. Here is my take on your first question:
I am currently not aware of any way to track the number of grown up kids in the way you want. However, if you have a school and all your children are attending it, each pair of students needs a house. If you want your town to keep growing, then use this equation (Number of Adults + Number of Students)/2 and you will have a very fast, but stable growth rate.
You can also look at "Number of Families" under the Town Hall, and see if you have enough houses. By what I know, pairs old enough to have children become their own "Family", and require housing immediately (if you want them to have kids).
I will now try to answer the second part of your question, please excuse me if my interpretation is incorrect.
It does not matter where you build a house as long as it has access to nearby food and firewood (or coal). It is nice to have it in an area where tools and clothes are nearby, so your people do not walk from one side of the map to the other.
Thus, I do not believe it matters where the children's house is in regards to their parents. If they have jobs that work in a similar location, then they might coincidentally be neighbors. Otherwise, they move as close to their job as possible.