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I'm just going to come right out and say this...
That's stupid. The often-used example of the flight attendant telling you to put your own mask on first before helping anyone else with theirs...completely true. You're not much good to your kids if you're dying. You can't pour water into someone else's glass unless yours is full.
I have Ehlers Danlos Syndrome. Because of it, I have physical limitations and often need to stop what I'm doing to rest because of the severe fatigue and/or intense pain. I explain to my kids that Mommy has owies and doesn't feel good, and that I need to sit down. They are accepting of that, and the older kids make it a point to help out with the younger ones when I'm not feeling well.