Plastic canvas is a good teaching tool for youngsters. It is larger and easier to work with. After some experience and success with this medium they will be ready for Aida and embroidery floss.
Not all children will take to cross stitching the same way. Some children will lose interest quickly in this quiet hobby, but hopefully will decide to pick it up again when they are older and have longer attention spans.
Stamped cross stitch will be easier for youngsters to follow than a charted cross stitch pattern. A stamped pattern will not require counting and will result in fewer beginner's mistakes and frustration.