You need to enforce your own policies. If you don't follow them, you can't expect your clients to.

When the mom sent the text asking about the children, that was the stage to actually say to HER "ARE YOU KIDDING ME??. Do they not READ the huge section about sending kids who have had symptoms, fever etc for the past 24 hrs??". Had you done that, and told her to get her tale out of bed and come and get her children before the infected the whole house, then the second occasion where they turned up with the rash wouldn't have happened.

If you aren't going to enforce the things you put in your contract, why the heck should clients pay any attention to them. You are in danger now of having set a level of acceptance and you need to rein that in hard and fast.

Have you actually said anything to these parents about either time? If not, why would they do anything different next time?