NTA you are not the asshole. It seems somewhat feasable and legit, if everybody agrees.

But still I recommend against the idea.

Reason: Conflicts between ex and gf will arise. They are unavoidable. Life in general will become more difficult for all. Life for the kids has just started to be more difficult anyway (the teenage years). In all this, I cannot help the impression that you are the kind that avoids conflicts instead of resolving them.