birthday advice

With the school year about to start, we have already recieved two birthday invitations. I don't know what we will do for the girls' party this year. They will each be in a different class at school--if we were to invite all the kids in each class and parents (which seems very generally to be the practice), we would have 100-150 people in attendance to celebrate a sixth birthday. That's the size of a wedding!

And, if we attend each birthday we are invited to, we would likely be going to roughly 50 parties. Any mother of multiples out there have ideas for how to best address this issue? Help!