What surprises me is that this was not a case if riding for play, it was their daily, routine, trip to school. The bike had stabilisers and I am wondering if they've given her the bike because she has complained of becoming tired out by the walking.
fwiw, because cycling is so much more energy-efficient than walking, she is burning off fewer calories by riding instead of walking ie she is getting less actual exercise this way, because the mileage is fixed (the idea with bikes is to travel far, after all).