My training teams always end up unbalanced

I would rebalance if the game has stopped giving both teams a useful problem. Leaving one side to struggle for twenty minutes does not automatically make them resilient. Sometimes it just means fewer touches and less learning. You do not have to announce a “better” and “worse” team. Move one player, change a neutral, alter the space or give the weaker side a temporary overload. I often adjust teams between short rounds rather than halfway through one. The question is: are both groups getting enough challenge and enough success to stay in the game? At U10 the coach controls the environment. Use that control. Players still need to cope with different teammates, but they do not need perfectly even teams or deliberately unfair teams every week. They need a game that stays alive.