What happens when two players both grab the ball?

That's a held ball: players from both teams have one or both hands firmly on the ball and neither can get it without being rough. Blow the whistle straight away, before anyone gets hurt wrestling for it, and signal a jump ball (both thumbs up), then point in the direction of the possession arrow.

The team the arrow favours gets a throw-in from the spot nearest the held ball, and the arrow switches to the other team.

The same applies if the ball gets stuck between the ring and the backboard.

In Year 3, blow early. Kids will keep tugging, and it's a common way for fingers to get hurt.