You can buy microwave popcorn that's just popcorn -- none of the seasonings, butter, etc. I'm not sure of the brand, but a heath food store or whole foods type grocery should have it. I put a little salt, and maybe pepper and parmesan cheese on it -- sounds a little weird, but I like it a lot. Another option is to pop it yourself on the stove: heat 1 T oil in a medium saucepan with lid. Drop a few kernels in to test the heat of the oil and when they pop, add 1/4 c popcorn. Cover, and let pop, shaking occasionally to shift the popcorn around. When you stop hearing pops, remove from the heat, add salt and enjoy. Not exactly healthy, but you can also make your own caramel corn. My mom would do it with brown rice syrup, but I'd imagine anything sweet would work. Heat the syrup after the popcorn is popped and then stir the popcorn into the syrup to distribute it. I think about 1/4 c for 1/4 c unpopped popcorn. You can add nuts into this too, which adds some protein.
For ground beef, a delicious (but definitely NOT budget) substitute is buffalo. It's got more flavor than beef, is leaner, and has more iron per serving. And, if you think your kids would have a problem with buffalo, you could always just... ah... omit... telling them that it's not beef until after they've eaten it.
