That's going to be an expensive one, sorry to say.
You have a lot going against you.. One of the premier teams in the NBA and its on a Sunday afternoon which is rare.
Celtics only have a few games a season that fall on a weekend afternoon which means only a few games where families can take their young children.
I'd just watch tickets through the ticket exchange and stubhub, and if a set meets your budget i'd suggest jumping on them.