re: I'm stumped by this tax question

I think there is some confusion between the old and new rules.

To get the 250K/500K exclusion, you have had to live as your primary residence in the house 24 of the last 60 months. You don't have to put the money into a new house. There is also no lifetime limit, however, you can only take advantage of this rule generally once every two years (with a few exception).

Now, as to the addition, who is going to "own" that? It sounds to me that perhaps the parents are gifting 100K to the kids. If that is the case, the parents, may, depending on how this is structured, be required to file a gift tax return. Assuming they have not hit their lifetime maximum for gifts, they would not owe any tax, but a return MAY be required. Just wanted to point that out.