If you're writing AS the celebrity then i think that's ok because it's basically just fan fiction.
As for just having them in your novel as regular characters, i don't know. I think there are issues surrounding that, but i suppose it's ok as long as you're not planning to publish.
And sorry, but i've never heard of a 6th grader that actually likes coffee, nevermind drinks it. But whatever, it's your character.
