A good rule of thumb with kid's bikes is that if they are small enough to fit in an average sized car boot they are too small for our bike carriers. Most (but not all) 16" bikes will fit in a roof mounting bike carriers. Some 14" bikes will fit fine but sometimes they will be too small. Fairly basic frame holder style bike carriers, such as the Thule FreeRide and Cruz Bici rack, tend to be better for smaller children's bikes.
Bike carriers