A fielder must be in the act of making a play in order to block the bag/plate from the runner(OBR 7.06 and FED 8.3.2). This can be an extremely fine line, depending on the circumstances surrounding the play. The rule of of thumb that was taught to me was, make sure you see what you think you are seeing. A kid trying to block the bag/plate in order to be a punk is guilty of obstruction. A kid blocking the bag/plate in order to make a play is guilty of good baseball.
Working within the rules to obtain an advantage is good. Crossing that line and breaking the rules is bad. . .bad sportsmanship, bad mechanics and bad for the game. As an umpire, this is why we constantly discuss, study and learn the rules and the spirit of the rules. Any knucklehead can memorize a rule book. It takes more than that to understand the spirit of the rules AND how to apply them. (I think Papa C harps on this).
As for safety, I refer you to page 1 of the FED 2002 rule book. . ."These rules are intended to promote and preserve the sound traditions of the sport and the safety of the participants. . ." If you are calling FED ball, safety and the act of preventive officiating is a fact of life that is pounded and pounded. FED calls this "Points of Emphasis". Of course, I have always wanted to ask, is it really "safe" to stand in a box and let some kid throw a 1 lb leather sphere at you at 70 miles an hour?
And when they start making kids were hockey equipment, I will park myself at the local watering hole and talk about how things were "in my day".