You can bump up the nutrition in the existing foods, and then start introducing new foods. Do not custom-make meals - teach the kids about new and exciting foods, get them to TRY them, don't put dinner away and jump up to make something else. Try making your own nuggets with chicken tenderloins (cut them up), dipping in egg and then some combination of whole wheat bread crumbs, wheat germ, ground flax seed, etc. Lightly fry in a little canola or olive oil to brown/crisp them, then finish in the oven. Use whole wheat pasta. Make rice or couscous (anything that absorbs all the cooking water) with "green" water from steamed veggies - I pour the water into freezer containers or ice cube trays, then use it as needed. Make "fries" out of sweet potatoes. Jessica Seinfeld has a couple of books about hiding nutrition in kids' meals - she uses a lot of vegetable purees (butternut squash gets added to mac & cheese, etc.).
Make really healthy muffins and pancakes - I put ridiculous foods in my son's pancakes, you'd be shocked!
My son also likes falafel - I use a mix but reduce the seasoning/salt by doubling the chickpea mix with more ground chickpeas, wheat germ, flax seed, and increasing the water with veggie water. He thought they were nuggets for YEARS!
Get the kids involved in going to the farmer's market or the produce section of the store, and select things. Let them help with food prep. Start a veggie garden in your yard if possible, or do some container gardening. A fresh tomato tastes a lot better than a cardboard freight-car tomato that was picked a week ago. Use any kids' cookbook to show how to make funny looking food - making faces with veggies and so on.
Kids who are 2 and 5 will not starve if they refuse a meal. Excuse them from the table if they are "done". In 20 minutes they will be hungry again, and you can present dinner again. If they still refuse, tell them dinner is over in 5 minutes. If they go to bed hungry, it's not the end of the world. Kids don't control what they are served - parents do. Don't give in and throw a yogurt at them if they refuse what you have cooked. I think of big families from a generation ago when there were no convenience foods and microwaves or fast food - you ate what was on the table, or you ate nothing.
But it's more fun and less stressful if you can just hide the nutrition, then introduce new foods in small amounts. Help play games identifying colors (for the 2 year old), get the 5 year old on board to help give the 2 year old better foods, etc.
You can also try a balanced food supplement (NOT vitamin pills and certainly not gummy vitamins - horrible) - my company makes a terrific one which is used all over the world and is often the only meal kids get. It's safe and carries no warning labels like vitamins do. It helps to make up the gap in nutrients even in our country, where the AMA said 10 years ago that we can't get what we need from our food no matter what we do, short of eating 4000 calories a day.
Try to make meal time a fun time where you talk about your day, tell stories or jokes, or whatever. Have an occasional picnic in the living room - put a big sheet out on the floor and use picnic ware, anything to mix it up. Try some 'make-your-own' meals like salad bars or taco bars. Tell the kids to pick any 4 ingredients. Kids also like to "dip" so make a healthy dip and let them dunk their raw veggies.