My bathroom is definitely open for them to use but other than those 3 times, (is it weird that I remember who they were lol), they don't use it. There's no need for them to use it really. They can go at work or at home. Twice was a mom who drives the longest (about 20 mins so not even that long and once she was pregnant) and once a mom and her school aged child used it.
If they were using it like yours though, I'd send home a letter too! I don't think youre the one being rude at all!!
I'd say, "due to recent findings in my washroom, multiple service calls for the toilet to be fixed and constant disinfecting, my washroom will now ONLY be available to the children during daycare hours. Should you or your older children need a washroom, there is a restaurant across the street. Sorry for the inconvenience but I thank you in advance for understanding the predicament I am in"
Since they all use it, I can't see any of them asking for details as they'll be scared it's directed at them. I'd assume they all read it and move on as they may feel awkward.....but oh well!!