The harnesses they use are safety harnesses where one is always locked on the cable so accidents are near impossible. They run a brief safety drill prior to you getting into the trees that explains it. As for your kids, I been there when other kids the same age as yours are present and they had a blast. For the courses I'd say stick with the yellow course as those are the lowest and not too hard for kids that age.
Personally I think if you stick with them, stay on the simply courses, and just encourage them to take risks, they'll be fine. Definitely have seen that age group on the more advanced courses before. Good luck!
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