ISO: youth 20 gauge, would prefer one with a scalable stock to make adjustments as my 8 year old grows, similar to the mossberg with the different butt pads. I prefer a full length barrel
Not sure if you want an auto, but I bought my wife a Tristar Viper G2. It comes with a youth and an adult stock. It shoots great and hasn't had any malfunctions in about 200 shells so far.
My son has a Maverick youth 20ga. Seems it was about $200 and fit what you're looking for. Has been a good little gun. In fact I kind of like shooting clays with it myself when we are out....