conuly

I think the correct response is to say to the women in a really sympathetic voice:

Wow, it must be tough with children that young to deal with them being so severely ill... My best wishes to all of you.

When they get confused and say something like "severely ill? No, they're fine..."

Then you say, Oh, I'm sorry - I just assumed since seats are usually reserved for the elderly or people with disabilities on crowded buses, and most kids have more than enough energy to stand up through a bus ride that yours must be severely ill for you to be having them sit through it.