My 2-cents is to make sure the bike fits, a lot of parents try starting kids on bikes that are too small or too big. They should be able to put their feet flat on the floor while standing over the bar. (Less of an issue for girls bikes.) And on tippy-toes while on the seat. Larger wheels are easier to balance than smaller ones, but it shouldn't be too big. We were lucky and got a hand-me down 18" bike from a friend. It's a trickier size to find, but it was the right size for both my kids when they learned without training wheels. And my son has a short inseam for his height so I've always had a tricky time finding him the right sized bike.
