On the tiredness -- you might start an iron supplement if you haven't already. Talk to your OB about something like Slow FE (you can get it over the counter at any drug store/walmart). While tiredness is certainly normal with 2 preschoolers to chase and a bun in the oven, it may be aggravated if you're anemic.
For easy cooking -- you should be very good friends with your crock pot. I've put chicken breasts in with a bunch of broth, or throw in a roast with beef broth and cream of anything soup. A recent favorite is:
Pork BBQ sandwiches
boneless pork ribs cooked in beef broth for about 6 hours on low,
about half hour or so before you want to eat, preheat the oven at 350,
take 2 forks and shred/pull the pork apart (it falls apart easily -- if your crock pot insert isn't oven proof, you'll have to transfer to a baking dish for this),
dump a bottle of Sweet Baby Ray's BBQ sauce over the pork (or the sauce of your choice),
stir it around and put it in the oven for 20 min.
Serve it on buns with a side of potato chips.
When I first quit work to stay home I had my 4 mo old son, my friend's 4 mo old son, and my 2 yr old daughter. I DID use nap time to cook dinner and it worked out great. Spaghetti meat and sauce can be prepared in advance, so all you have to do is boil pasta when your husband gets home. Home made pizza can be prepared at nap time (cook the meat, chop veggies, prepare the dough), then throw it all together and put in the oven when hubby gets home.
Also, I wasn't above putting kiddos in a playpen or behind a baby gate for 20 min so I could get dinner ready without them underfoot.
Hang in there!