I am not a big fan of most of the "youth" crossbows out there. They have sub par components and corners are cut to bring down the price point.
I am a fan of full size crossbows that have adjustable buttstocks that the kids can grow into, then they can use them as teenagers or adults if they wish. My favorite is the Mission MXB320 for kids. It is a light crossbow, quiet, great components and the buttstock can be adjusted 3 inches.