Birthday party questions?

How do you do it when you have a party at a location and pay per kid? We are planning Lexi's party at an indoor play place and with the people we know and the ones from her class that she wants to invite there are 22 kids. When addressing invites for the ones in her class what should I do? I do not know these parents and do not want to pay for siblings. This will be the first time we have done a location party, so I do not know what is proper ettiquette. Am I supposed to pay for siblings of her friends too. We plan on paying for all kids in families that we know...but for her classmates what is the standard? We have no idea, but that would add quite a bit extra to our costs.
Any insight is appreciated... This would be a win win for me to have it at this place...no clean up before or after...they provide entertainment, food, all paper products. We just show up, play, and leave them with the mess. Fantastic for a hugely pregnant, busy mama.