Always good stuff to think through when taking the kids rafting. I have two priorities when I take my kids out (ages 2 & 4).
1) They must be safe.
2) They must enjoy themselves.
Kids won't enjoy themselves unless they feel safe. I always make sure that my kids have good solid handholds wherever they are in the boat, and that an adult is within arms reach of them. In fact we are still holding the 2 year old through most rapids still.
I agree with the sentiment of keeping entrapment hazards to a minimum. However, since I'm not taking the kids on anything above a Class III with very
minimal chance of any kind of swim/flip, I usually opt for the extra strap as a handhold. Easily adjustable and movable to whenever the kids want to sit next. My kids like to move around a fair amount and don't like to sit in the same spot for too long.
My 4 year old's favorite is to straddle the tube and lean forward till her chest is also resting on the tube. She grabs the chicken line with the outside hand and I give her a strap hand hold for the inside hand. Super secure position for her through the rapids and she still gets splashed a lot