The responses so far are focusing just on the "mean girl" situation but I think there is a much bigger issue here. Your child was in a public place (and...why exactly is the school having a so-called official function in this place and there is not one school official or teacher present?). The other adults are completely in the wrong for having left children not just unattended but in a place where the adults could not even see them -- for two solid hours!
Any child could have wandered off; gone out a door; been accosted in a restroom; been persuaded to come outside by a stranger. Yes, these are 4th graders and not toddlers, but it was still utterly inappropriate for the attendant adults to abdicate all responsibility. Just because it's "a Chuck E. Cheese" type place and there is an employee around does not mean the parents were allowed this little time off they took in another room. The employee is not responsible for, nor could she have kept an eye on, every single kid. (Someone posted that these types of places are where adults can relax and kids have free rein -- These aren't babysitting businesses, people. The kids still are the responsibility of the adults who brought them.)
Your child disappeared -- to the point that another parent had to call you and assumed your child had left the building! That would worry me FAR more than the very typical kid spat being talked about here.
If I were in your shoes, I would ask for a meeting of your PTA or whatever the parent organization is at this school and say it's time for a policy that when a school event is held, parents and/or teachers must be present in a certain adult-to-child ratio and actually in the same room as the children. The moms who were at this event will be miffed but point out to them: They thought your child had LEFT. If they'd been in the same room paying attention to the children they could have redirected the mean girl and might have known much sooner when your daughter was left alone on the playground.
There is a fundamental safety and responsibility issue here that would have me hopping mad, frankly. Your child could have decided she was done with this party and that she would try to walk home, or walk to a phone and call you. It's happened around here -- a kid who was not happy at a party and just decided to walk home, sparking some terror for the parents. The result ended up OK, but anything could have happened.
Even the most responsible, level-headed kid, in a situation where she's being bullied or is unhappy, could make a mistake like that. And it's up to the parents to BE parents--for all the kids, not just their own--and not assume, "They're fine because they're in this kind of place, and there's a staffer in there." The adults could have taken it in turns to be in the room with the kids.
I'd really get involved and insist on some policies for the future.
