Should defenders swap left and right sides?

Yes, I would give them experience on both sides. A preferred side is completely normal, and some continuity helps defenders learn relationships. But swapping occasionally creates different receiving angles, tackling pictures and opportunities to use both feet. I would not change them every ten minutes just to tick a rotation box. Use blocks of games or training periods so the player has enough time to actually learn the new side. You can also ask them what feels different. The left-sided defender may receive with the touchline on a different side, see different passing lanes and defend different body positions. At U12, I’d keep the preferred side without making it permanent. The aim is to develop adaptable defenders, not make every player equally comfortable everywhere immediately.