You know...if it were me, I'd be really tempted to reply, "okay...exactly whose eating opinions should be forced on them; mine or the commercial food industry's? Because if I don't teach them what healthy eating is, television, magazines and billboards will have no problem teaching them their version."
It really does depend on how you present it to the kids, but I do wonder exactly what your in-laws think you're depriving your kids of by cutting back on junk food?
If you want a strong, quality home you don't use cheap junky materials to build it. If you want a strong, healthy body, you don't use cheap junky materials [food] to build that, either.