I really feel for you. When my kids were in that age 3y. 2y. and new baby, I was so drained that I was really just functioning by forcing myself to do whatever needed to be done. I had no energy and it was not enjoyable to do things with my kids.
We had a very mainstream diet.
Now they are grownups and I am 60 y old and I have way more energy than I did back then when they were little. I actually take care of my grandchildren quite a bit and I enjoy it very much. I can keep up with them and have no problem. At the end of the day I am not drained and when dd is at our house with the kids, I get up with the older one (not nursing) at night if need be.
But our diet has been very good for many years now.
NUTRITION is EVERYTHING. Believe me, that is really it. I don't know what your diet is like but here is what I would suggest:
Cut out all sugar. All sodas, all junk food, all fast food. And cut out milk. You can have cheese and yogurt but avoid milk.
Eat only whole grains, legumes, nuts, seeds, fruit and vegetables. Drink only water or herbal teas.
Try and take a nap when the kids are napping. Even a few times a week will help.
Your diet can determine how much energy you will have. But of course sleep deprivation is also a major factor. But not as much as diet. You can do with a lot less or interrupted sleep if you have the right foods.