A.S.
I agree, you should definitely speak to the family when planning this. I am a fundraiser full time for a non-profit organization and there are quite a few things that go along with planning a benefit. First, the parents need to look into setting up a 503C status. Any funds they are given will need to be claimed on their taxes and that could effect them so they should talk to a bank or accountant in setting up a separate 503C which would create a "foundation" in their childs name so that they can avoid being taxed.
Also, if you can find a venue that will donate the food and space that will save you tons! Otherwise you'll probably have to pay per person and then add an additional fee to the ticket price in order to profit at all.
Silent auctions are a great way to make $!
I would strongly suggest forming a small committee so that you do not have to undertake all the responsibility and then each person can use their business contacts-connections to get items donated and possible find a venue or local dj willing to donate services.
You can always donate the proceeds to an existing foundation, such as the hospital the child is getting treatment at.
Sorry if this is overwhelming, just wanted to give you some info!
I am happy to help with any questions you may have!