There will come a time when young keepers end up on the floor at the feet of an opponent. The referee can protect them after the play. But during the play, the keeper has to protect themselves. This post explains how.
Initially the keeper stands opposite the coach with a ball in their hands.

The keeper passes the ball through their hands, to gently roll behind them.

The keeper then turns, and dives on the ball.

The keeper should secure the ball close to their chest, and tuck their head in, while allowing the upper arm/elbow and leg to rise slightly.
The coach will then, taking utmost care, swing their leg and foot over the keeper, being careful not to make contact with the keeper.

The ultimate objective of the drill is to train the keeper to protect themselves when on the ground. If they can grab the ball and make it secure, they will reduce the risk of being kicked.
It’s human nature to want to kick out at a ball that is “available”. Try it yourself. Have someone hold a ball securely in their arms. Then put the ball on the floor between you. You will feel a natural urge to engage with the ball. Similar feelings occur when the same ball is held out towards you. The holder still has contact, but the ball is now much more “available”.
The key is making the ball seem secure to anyone in “kicking distance”. Even one’s own teammates will want to kick the ball way. To make oneself safe, one has to make the ball look secure.
Once the ball is secure, take extra precautions. Raise an arm or elbow in front of your face. Lift the upper leg a little and tuck the knee into your belly. Make sure to keep the foot pointed down and way from other players. The referee has a duty to protect them as well!.
Sure both the elbow and the knee can get kicked. But this is far better than taking one in the face, eye, or groin.
Above all, remember, no one wants to kick the keeper, the referee will protect you to the best of their abilities AFTER the play. See an earlier post here on possession – when the keeper has control of the ball and others must not attempt to kick at it.
But during the play, the keeper has a responsibility to protect themselves.