I totally see what Margy is saying and that makes perfect logical sense, but in fact you could run afoul of the ADA if you refuse care to her because of her period.
What you COULD do is set a weight limit to the kids you care for, due to you own abilities to pick them up and for safety sake so they don't fall, and weigh her, and declare her overweight for what you can safely manage.
Of course, you will have to hold to that weight line. Do you have heavier children who you care for?
As many positive changes the ADA has made for americans with disabilities, they have shut down services in other areas because not all people who serve can serve every single person. I had a nasty experience with the ADA as a volunteer working with pregnant moms at risk, and it has left such a bitter taste in my mouth that I feel the need to warn you. It's like, if you serve people with this condition, you have to serve EVERYONE with this condition even if it will bankrupt your business.
Best wishes. I can feel the empathy you feel for her and her family.
I think you are wonderful to look after these children I know it cant be easy ,I would say telling the truth will work best, saying it as it is, the way you have done here, cant see that would offend anyone or any 'act.tell the mom it simply ,just the facts and give her some time to find other help . Good Luck