Oven-friend potatoes are easy - just have them cut washed but unpeeled potatoes into more or less equal sized wedges. Red potatoes are usually rounder vs. oval, so those can be cut into halves or quarters. Put them on a baking sheet and drizzle some olive oil on them & toss to coat lightly. (They can use their hands and just use dish soap to get the oil off their hands - that keeps them from using a spatula and potentially launching potatoes all over the counter as they toss!) The oil also keeps them from discoloring even if you don't bake them right away. If the kids are willing to peel some sweet potatoes, add those in or just make them by themselves. They can add some dill or Italian seasoning if they like it, or just some salt & pepper. Bake at 350 for about 20-25 minutes.
Quinoa is an easy side dish and breaks the brown-rice-boredom you mention. It's also loaded with protein. It comes in various colors - let the kids pick out what they want. Couscous is super easy and fast, and you can use the whole grain version. You can always use water, or you can use broth (chicken, vegetable, etc.) for all or part of the liquid. If you use chicken broth, use low sodium/low fat and no MSG added.
Falafel is a little more involved but my son loves it. We used a mix but cut the sodium by adding some wheat germ, ground flax seeds (meal), sesame seeds, and additional water or broth to reduce the sodium content (one mix but twice the number of falafel patties). Have them make the mix first to see the consistency, then add more of the other ingredients to stretch it/dilute the salt but still keep the same consistency. You don't have to be all that exact. We make little patties (round like meatballs, then flatten as you would a burger) and they are about 1.5 inches across and 1/2 or 3/4 inches thick. We quick fry in some olive or canola oil until they are crispy on both sides, then finish in the oven as you would potatoes. You might want to do this with them the first time, but falafel is delicious and has protein in it from the chickpeas. They can be eaten as a side dish or put in pita bread, or put on top of a salad. Tahini makes a nice dressing but it's not necessary. I used falafel to hide all kinds of nutrition from my picky son - I put drained, thawed frozen spinach in there (he thought it was parsley), pureed broccoli, and as I said flax seed and wheat germ.
If they like Mexican food, have them open, rinse and drain some canned beans (any color), chop some green peppers, onions, etc., open some grated cheese (cheddar or taco/Mexican blend), defrost a handful of frozen corn or peas or both, whatever they like. If you have some leftover hamburger (cooked) or some cooked cubed chicken, use that up too. Open a can of refried beans. Use a package of whole wheat tortillas, spread a stripe of refried beans down the center, top with a few of all the chopped veggies, a tablespoon or two of the beans (any variety or combination) and some cheese. Fold the 2 sides of the tortilla over the pile of fillings and fasten with a toothpick. Put on a baking sheet spritzed with baking spray, top with some enchilada sauce or some tomato sauce (just plain, not spaghetti sauce - they can add some cumin or coriander powder, or cilantro flakes if desired) and maybe a little more cheese. Bake for 20 minutes to heat through. If they make their signature rice with half tomato sauce/puree and half water, they'll have a Mexican side dish! I use regular brown rice (not instant) and get it started, then make the enchiladas in about 20 minutes, let them bake for 20 minutes, and everything is done at one time. (If you've noticed, the kids have just made the whole meal!) They can do this assembly-line fashion if they can cooperate with each other!
If you like to grill, they can do potatoes or various veggies on skewers. They can cut peppers and onions into big chunks, do potato chunks or small fingerling potatoes whole, mushrooms, zucchini rounds, and so on. I don't alternate so much on one skewer even though it looks pretty - I don't think the cooking times are the same. I do potatoes on one, peppers (any color) and onions on another, and mushrooms separately. Zucchini can go with mushrooms or peppers. This puts the kids in charge of starchy and "green" sides. (And the adventurous kids can also thread chicken chunks onto skewers too as you "promote" them to main course!
Once they get good at cutting veggies, they can do stir fry as MommaL said - sautéing is easy and everything goes in one pan except the rice.
Finally, I want to compliment you! You are doing your kids a HUGE favor by giving them these skills! My son learned how to do this sort of thing as well as do his own laundry, and it helped enormously when he went to college and had to fend for himself. A lot of his friends still thought, even at age 18, that the garbage and recycling takes itself out, the dishes wash themselves, the clothes fold themselves and jump into the dresser drawer, and so on. He felt so competent compared to some of them! These are life skills they will need!
You'll find that kids eat a more varied diet if they make it themselves. If you can take them to the grocery store with you, to let them pick stuff out, you will do even better. If they're invested, they'll work at it more and actually eat it too. There will be less waste as they see the frustration of working at a dinner that no one eats.
Good luck!
I forgot to say that they won't use all of the canned/rinsed beans in one meal, so those can be frozen (together or separately) and just popped out/defrosted the next time around. I do that for the whole beans as well as the refried beans, using appropriately-sized plastic containers (for everything) or ziploc bags (for the whole beans).