Hmmm tough one. The way I do it is if one of my kids is sick ( I mean SICK, like vomiting, not just a cold...) I tell the parents about it and give them the choice. Now I have lost income because of this, but here's the thing; it doesn't take long for parents to realize that even though they tried to limit their kids exposure, the child was already exposed anyways, and then they end up having to take a whole lot of time off work. So now, when I send out an email about my kids being sick, most parents just bring their kids in anyways because they just assume that they will get sick no matter what. In your case, it's really up to you...but measles sounds like a bit more of a serious disease...if I was a parent I would be concerned....if she really wants the day then give it to her, and next time be more clear with your parents about whether or not they have the choice to pay or not.