Fill one bowl halfway with water. Place an empty bowl next to it. Put a small sponge on a tray. A folded dish towel underneath catches drips.
Dip the sponge in the full bowl. Squeeze it into the empty bowl. Repeat. Don't say anything.
Total concentration. This is the kind of activity Maria Montessori built her career on. Some children will do it for 30 minutes. They will absolutely spill — that is part of the work; the towel handles it.
Color the water for visibility. Use a turkey baster or a small dropper instead of the sponge. Add a third bowl for "transfer back."