Go with more hobby grade than toy grade as with the toy grade, once something happens to them you might as well toss it in the trash. Hobby grade you can get replacement parts for if or when needed. They'll also be much faster and quality a lot better.
Don't underestimate 8 year-old's. My son was racing and winning at 6 in RC trucks in the Novice class against people closer to my age then. Now that he's all grown up at 30 he's still racing gas boats with dad. Look at Pro Boat or AquaCraft as they both have hobby grade boats for a little more than $100.00 as it'll be worth it in the long run.