Angela says she is prepared to spend a quarter of a million dollars for Billy's birthday. This kid isn't family. They never had any birthday parties for Jonathan nor Samantha (at least, never aired) and, all of a sudden, this boy is more important to the odd non-couple than their own children ever were. This makes no sense.
The last time the Harpers were shown, they had a daughter Jonathan's age, but no son. It hasn't been long enough for them to have had a son the same age as Billy.