A.H.
There is a petting farm in Irving (I think Fritz park) or something like that. Not sure if it is too far. We had one at the Ark in Coppell and they were open to me bringing decorations, games, pinata, crafts and (of course) food. It was cheap, but you have to book early (like now). It was a good indoor party.
Good luck!
You could also try to contact one of the Flower Mound pumpkin patches- maybe they would rent their area to you. (Not sure either one is usually open in the spring) Hey, it never hurts to ask.