Before you buy them new bikes, take the pedals off their current bikes and let them learn to balance on the bike by scooting around with their feet (like those tiny little bikes with no pedals for small children). Training wheels are a terrible way to learn how to ride because all the balancing part never gets learned and it takes hours of you running alongside the bike holding the seat.
My 2 cents coming from someone that has taught five kids to ride a bike.