Choosing a kids fishing rod that is durably adequate for a child (particularly a destructive child) yet lightweight enough for them to carry and cast is the key to finding the perfect balance. A 12ft surf can often be hard enough for beginner adults to cast, so it definitely won't be suitable for a child. Yet you also don't want to hand them a rod that is too short and shirk their confidence.
Starting the young one's off with a three to four foot fishing rod allows them to be comfortable yet still get the performance they want from their combo. As your child gets older and becomes more experienced, you can adjust the rod length accordingly. As for material, something durable such as graphite, fibreglass and composite blends can go a long way to extending the life of your child's fishing combo.