FIRST and SECOND GRADE, and you're asking this???
Play the damned game any way you want.
Then, go home and think about what you are doing to those kids. They don't need to be taught competitiveness, it happens all by itself quite nicely. At this age, they need to get out and play for fun. The ALL need to exercise and run and do all those sport-type things, and no one should EVER be on the bench any longer than anyone else.
The good kids should be thrown off the team because they skew things for everybody else. Ages 6 and 7 are too young to be doing all the complicated things that it takes to win consistently at this kind of game. Instead, they should be building up a base of fitness, cooperation, rudimentary rules, learning to take coaching, and good sportsmanship. It's much too soon for the kind of physical stress that comes with specialized positions and overly intense competition.
These are kids, idiot. They aren't grown enough to know who will be the right size and athleticism when they ARE old enough to play competitively. Your good kids will probably "burn out" and the ones that will turn out to be the REAL athletes won't play because they had such horrible experiences at this young age.
Can you TRULY go to sleep at night knowing that you have given a safe, appropriate, healthy play experience to EVERY KID on your team AND on the teams that they play against?