J.C.
You are not being too strict at all. A sick little one needs to stay at home or else she needs to set up a family member as backup that will watch him when he is sick. I have worked in daycares and schools and know that a lot of parents will send their child sick because they can't or don't want to miss work. I'm not saying that mom does that or not, but the bottom line is that you are 100% right to not want to watch the little guy on those days when he has fevers, rashes, etc. That's what is best for the children, even though it is inconvenient for her, and she is his mom so she has the responsibility to take over care when he is too sick.