I dont think you should. The hike itself can get really intense unless you or someone are experienced hikers.

You can. But you wouldn't be able to do the high jumps

Yes but I would suggest not to because it can get really slippery and dangerous at times. I didn’t see a lot of children while we were trekking the jungle.

Not sure what the official rules state, personally I'd say yes if you want to have lunch by the falls but no if want to do the full canyoneering route.

I wouldn’t, for an adult I felt the most dodgy part of the trip was the 3 up motorbike ride up with only a climbing helmet as concession to safety. The guides were knowledgable and helped less able members of our party through the trickier bits but a 4 year old would find it scary I think. Also, once committed to the canyon there’s no easy way out