I would just tell them the horses you have now are too old or too "excitable" for a beginner rider.
I wouldn't let any child ride Cookie even if she wasn't a spaz half the time. I don't even let anyone walk my dogs but me because I'm a control freak when it comes to my animals.
Adult humans are stupid around animals most of the time without even trying to be. Kids are worse.
No riding of the horses would be my answer. If they really want to ride why not just take some field trips to a place that does trail rides?