My son just turned 6 and has outgrown his first bike. Since he's just about 4 feet tall, we've been told that a 20" wheel would be the most appropriate size for him, until he reaches 5 feet. My question is this: Are the bikes that one can purchase from the big box stores -- Wal*Mart, Target, Toys R Us -- significantly enough inferior to or somehow more "unsafe" than what one could buy from an independent bike shop that there would be a concern that the bike would not last long enough for my son to outgrow it? I'm concerned about shelling out money for a product that a 6 year old is going to be using and eventually outgrowing, much the way I would feel about buying a toddler designer label clothes that get outgrown too fast and stained on first use. I'm interested in reading what others have to say, thanks.