....another thing, as a mother I wouldn't allow my ex to bring a gf I didn't know to stay around my kids either.

The kids are going to have enough to deal with seeing a man they don't know, and trying to get their heads round him being "dad".
They don't need the added complication of a gf as well.

Not a personal thing, just a child welfare one.

This situation is going to be in his life forever, you should perhaps have a long hard think about wether this is something you feel able to deal with.
Best of luck going forward.