The trainers have the children warm up in a fixed pattern (round or square).
The children run behind each other with some distance between them.
The child in front receives the ball from the trainer.
Variant 1
While running, the ball is passed to the next person in line by means of a throw (short pass), the receiver passes the ball to the next person in line.
The person in the back passes the ball back to the last thrower and the ball travels the whole route in reverse order.
Emphasise technique where necessary and ask which form of passing is most effective.
Variation 2
Similar to variation 1, but now the ball is passed backwards without looking (i.e. by feel).
The emphasis now is on catching and uncatching the ball.
The last player in line to receive the ball sprints with it to the front to start a new round.
So there is always someone else in front.
Extension: To make it a bit more difficult, you can appoint 1 or 2 defenders who make it more difficult to throw and try to intercept.