I'm trying to think like a 10 year old...

Maybe take her shopping for HER pdf. Find a selection of USCG approved vests for her to choose from (while you have secret control over the selection).

a good pfd explanation is here http://www.waterways.nsw.gov.au/pfd.html

I think type 1 is the most age appropriate. 2 and 3 are certainly more comfortable, but this is a parental call

Nacra 6.0NA to kill myself quickly and a Johnson 18 to do it a little slower.