You're not overreacting (and I DO have children!) It's your house, your rules, and both parent and children were rude at not respecting the fact that you shouldn't touch a person's belongings (or animals!) while doing a job in their house, and that when the owner says no, it means NO. I would be really blunt- you do not want the pets touched, PERIOD, and if it happens again there are other cleaners out there!
Edited to add, I'm horrified at how some people allow their children to treat animals. At our cottage there is a public beach with a swampy area over to one side where there are quite a few frogs- there are always kids over there catching them and then throwing them, shoving them into tiny bottles, leaving them in the sun in buckets to die in the heat, etc. Their parents just sit there and watch them do it. By the time everyone goes home, there are dead and injured frogs all over the place! It drives me crazy... who would just watch their child terrorize or maim or kill an animal and not interfere? My kids liked going to the swamp to find frogs too, but our rule was always that we catch them if we can do so without hurting them, then we look at them and learn something about them and then we put them back in their home where we found them because that's where they belong.