How do I help children who think they don't need to know math, still learn math and have fun doing it?

I actually assissted in a math classroom for one of my internships to become a teacher. My lead teacher pretty much handed over the control of her "resource class" (those are generally the kids that have the hardest time with math, hate it, and think they don't need it) What I did with them that worked really well, was to present them with a project that required them to research their intended career field and find the math they WOULD need to use for that career (I seriously cannot think of a single career that would require absolutley no math at all - and even IF there was one out there, there is still shopping to do which involves adding, subtracting and using percentages at least).

It simply eliminated the argument, "but I won't need math."

Additionally, I talked to them about how there is an element of "learning to learn" involved.

Then, I did my best to make the learning fun and relevant as much as possible, games, graphing stats about the popular movie that was out, doing surveys and calculating facts and figures related to the survey outcomes . . .

Since you are advising and not actually doing the teaching, I understand the actual classroom apporoach is probably largely out of your control - using this approach might be somewhat difficult. However, I think the general idea still applies. Sit down with them and talk about their career plans and which math it is they will need to use for that career. Say all the things you are already saying. Play a game of poker (for cookies or something) with them and talk about how understanding probablilities helps their game. Then, if they don't take your advice - unfortunately, there probably isn't a lot more you can do for them. They'll miss out and either learn they are missing out, or maybe it won't be a big deal for them later (that will all depend on the individual).

Good Luck!