Did this excursion with my wife and another couple from the Triumph Apr 5-12. It was great fun. Half the fun was the van ride up into the mountains--lush scenery and one-lane road (with infrequent traffic both ways). Had no major scares on the drive.
My wife is very leary of water, and she had no problems. There are sufficient "guides" with you that you never get out of control. The river is very clean and scenic, never too fast-moving. They provide everyone with life vests and the tubes have bottoms in them. The only caution is that you are not supposed to brush up against the brush on the banks, as bamboo leaves can cut. At the end of the tubing (about 1 1/2 hrs on the river), there is a stop where you can jump into the river on a rope swing, and refreshments are available. You get a free beer or soda.
I don't think your kids will have any trouble--they'll have a hoot! I would check with the Cruiseline to make sure there are no age restrictions--I saw kids on our trip, but I'm not sure of their age range. Gor for it--Tim