First kid, first bike: New from REI
First kid, second/third/forth bikes: used from craigslist or FB marketplace, then sold via the same when outgrown or needed to upgrade.
DO NOT buy from: big box stores (target, walmart), or even Dick's.
DO buy from: local bike shops (used), Phoenix Bikes Co-op in Arlington, brand names only from FB or craigslist if you know how to measure the bike and know what brands come with first class QUALITY components, that are repairable (unlike some of those from WMT and TGT)