Youth 22rf.
What I chose in the late 70's might not be appropriate now, but for what its worth, heres what I did and why.
I started each of the kids shooting with a Rossi Youth Model Pump 22. Cut the stock down by an inch or two to fit them better. As there was no safety on the little Rossi, it was easier to impress on the kids that the only 'safety' they could depend on was between their ears. The external hammer, the lack of a mechanical safety, take-down capability, and the ability to use sub-sonic 22 Shorts sold me on the Rossi Pump.
After going through our three kiddos, the little take-down Rossi is about to make its way through the grand-kids.
I'm sure your situtation is different and there may be more options out there now, but the Rossi seemed to me the most appropriate platform available that would let me teach safety and responsibility. That was my goal.
salty.