You sure DID do a lot yesterday...too much! You have to pace yourself.
Children should not have to be entertained all of the time. They need to learn how to do things by themselves and learn how to use their imaginations. When my children were young, I had a child's table with coloring books, crayons, manilla paper, colored paper, blunt scissors, glue. puzzles, play dough, and picture books on it. They would go there to color or create things. Of course we had toys and the good old cartoons. I also would read to them and that takes very little effort. So, if your niece brought some toys over for the kids, you might be able to help. On slower days, you can do some craft with them or have them help bake cookies. And, in the not too distant future, the kids can play together and play board games too.
Now, I'm not remotely suggestion you do this every day but perhaps once or twice a week to see how it goes. This could give your niece a break. Just don't do your grocery shopping or anything else like this on the same day...at least not while the kids are so young.
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God does not give us a spirit of fear, but of power and of love and of a sound mind. 2 Timothy 1:7