We always tip. The places we've gone - Chuck E. Cheese, a plaster place, bowling, sports complex, gymnastics, etc., have always provided a helper for the party (in some cases we pay a fee for the helper, usually $15). It's always a teenager, and this person wrangles the kids, cuts the cake and pizza, sets up the tables, etc.
We usually give the helper $20-$40. The parties tend to be in the $200 - $300 range depending on number of guest and venue, so I think that's reasonable. I'm sure this kid is getting minimum wage, and $20 is always very appreciated.
I don't think the helpers are upset if they don't get tips, I just think it's a pleasant surprise. I wouldn't feel bad OP, if it didn't occur to you it's probably not that common in your area. You would have seen other parents doing it at their parties if it was going on.