It depends. There are plenty of moms who would stay with their child at that age, and plenty who might drop off as well. A lot more probably depends upon the specifics of each individual student. There are families that we were close enough and knew well enough that I would have left my child at their party earlier than 7. There are some that I didn't feel comfortable enough (either in knowing the families or the venue it was held at) to leave even at 8 or 9 yrs.
Where is the party, at their house? How well will it be supervised? What activities will be going on, swimming? If swimming, then no way would I leave my kid. If they are playing outside, that would be different. Will the friend that you both know be in attendance? Could you stay for a short while and see if things meet your comfort level enough to leave? I've done that too: Tell host and child (yours) that you may only be able to stay for a short part of the party... you've got some other things going on and may have to leave after just a short stay. When you get there, hang out for a bit and see how it feels to you. If you think it would be ok, then you can speak to your child and the host and let them know that you need to _______(run drop your daughter off at X event) and then you will be back, if that is ok with them. Leave your cell number in writing.
OR, if you get there and decide it isn't being supervised to your liking or see something that makes you wary, then stay for a short bit and then take hostess aside and let her know that you are sorry but can't stay any longer--you have to _______ (sister has to be somewhere for X sports).
Any chance their dad can take one or the other so that your son doesn't have to miss out? These kinds of parties (especially early in the school year and at this age) are excellent opportunities to meet other parents and get to know them. Often the parents hang out in their own little groups and chat at the party, while staying out of the way and keeping an extra eye out on the kids.