I spent about 2 weeks at the Hilton in June. I didn't notice any problem with noise or anything, overall it was quite a nice hotel. I did have some issues with the air conditioner in my room but I imagine that that's common in an island where most places run the AC 24x7!
9 days may be a bit much if you're just going to be hanging out around there; if you were really into scuba I would recommend staying as long as you can, but with an 8 year old it may really depend on what you're looking to do. Guam in general didn't really strike me as a very kid-centric place, with most of the activities along the Tumon strip being aimed at older Japanese tourists and US soldiers.
But if you're just looking for a nice place to relax on the beach and see some interesting things, it will be a good place. Just don't expect disneyworld-style entertainment for the youngster!