Does your 7 year old follow instructions well? Is he inclined to stay calm in an emergency? If he does fall out, would he remember and do what he was told (something like to float on his back with his feet downstream, for example)?
I would have taken my oldest on those types of rapids at that age, but probably not my middle child.
I would call the rafting center and discuss you concerns with them.
I loved the few times I've been whitewater rafting and look forward to taking my kids at some point - I was considering taking all three on a class II trip (ages 5 and up permitted
) this summer, but the cold water temps in Washington state (where we are going on vacation) have me reconsidering.