High school: 5C/5B, switch to 3C when developmentally ready. Some advanced players will wind up on a 1.5C by end of their senior year.
Buy Bach mouthpieces. Blessing may look the same, but the plating is much, much weaker/thinner. It wears off in a year or two, then your kids have raw brass on their face. If you choose to buy off-brand, investigate first.