I like that idea, Jennifer--parental permission slips. Nice transparency.
Some of the sites do say "or with parental permission," which would support Jennifer's idea. Something I was going to try. But some sites, particulary sites that involve communicating with others, are pretty clear about age requirements. I think we do our students a disservice if we "bend" the rules.
I haven't encountered this problem in the past, but was going to use Prezi this year and realized it had an age limit. So, my decision was to do permission slips. If parents don't want them to do it, they'll use powerpoint and if they do, they can use prezi.