We did plaster of paris masks (casts of the kids' faces) for dd1's 9th birthday last year, and was a fantastic success.
It is messy, but very easy and you get quite impressive masks in a fairly short time. But they will need to be dried and finished at home. This didn't seem to be a problem -- we gave the kids some spare bandage to take home with them. You do ideally want a couple of adult helpers, though, depending on how sensible/competent the children are.
If you want proper instructions and a list of materials, I can probably dig it out.
There was a basket weaving event on at our local library a few weeks ago - dds loved it. dd1's 7th b'day is coming up, and I thought about 'phoning to see if one of them would come round and help the children make something.
Totally agree that arty/crafty parties are brilliant for this age group. Whatever you choose, I'm sure she'll love it.
frogs, that sounds totally terrifying, but if you do get a chance to dig out the instructions I'd be really grateful.