Bring earplugs.
But seriously... I wouldn't worry about the head massage thing (or hand massages), as I doubt any of them would be that interested. Getting that age group to sit still for any more than 10 minutes at a time (especially in a group) is near impossible. Plus, it would mess up their hair - if they were getting the "glitter" style and treatment, they won't want you to mess it up.
Also, I'd nix the footspa idea - too risky. Not enough time to disinfect it between each user, not to mention the slip/fall hazard and the fact that verucas and warts are really prevalent in that age group. When I'm working on one adult at a time, its a different situation than trying to wrangle more than 3 girls (how many are coming to this party?
Keep things simple, and make sure you've got some quick-dry spray so they don't wreck their newly painted nails. (can you have a dvd playing in the background so they can have some "quiet time" after you are done with them?) Nails, sparkly hair, tattoos - even with help, you'll be hard pressed to do 8 kids in two hours. With this age group, I run a colouring contest to help keep the other girls busy. But if you have your head down to do their nails, you can't supervise the rest of them so make sure that's covered.
Also, definately do a search on this topic - we've discussed this a lot, so check that out first. (oh, sorry - I just saw that you did that... there's so many threads on this so you should find loads of ideas.
One last thing -
permission forms! You will be working on minors, and you should have permission forms signed by the parents outlining what sort of treatments you will be doing. Include a question about allergies/sensitivities/skin conditions as well! Definately CYA...
Good luck, and bring some paracetamol for yourself for afterward
* ETA: I just read this, and hope I don't sound too negative! I've done lots of these, and they can be fun, but they are a LOT of work, and you might want to strangle some of the participants by the end. My first one seemed so disorganised, and I tried to do too much. Start simple, bring help, and work from there. And let us know how it went!