One advantage is in my opinion the use of commercially available AA batteries. Here you can also use rechargeable batteries, a suitable battery charger you anyway mostly because of all the other children's toys in the house.
The disadvantage is the cost of replacement brushes over time: These are firstly not cheap and secondly they also wear very quickly. The brushes for the adult model last much longer!
TIP: If the child is something greater and the second teeth come through, just use the brush of the adult model, are indeed identical except for the color and the bristles!