I bought my 9 year old grandson an 870 youth Jr. last summer. It's exclusive to Gander Mountain with a 12" LOP stock and 18" barrel. It fits him well and he can handle it. I'm only letting him use it as a single shot for now so he learns the discipline of making good shots on game but it functions just fine.
Another benefit is that the gun can grow with him. As he gets taller the youth stock (13" LOP) can be added as well as longer barrels.
All that is neccessary for evil to triumph is for good men to do nothing