We are using bigger goals than the 9v9 recommendation

The recommended U12 and U13 goal size is 16 by 7 feet, and a larger goal does change the game. Shots that would normally be manageable become goals, the goalkeeper's starting position feels more exposed and defenders can begin protecting the goal rather than defending the football. I would raise the equipment issue with the club, venue and competition rather than asking the players to solve it every week. They can confirm whether the fixture should proceed and what permitted alternative is available. On the day, avoid judging the goalkeeper against an unsuitable goal. Keep the coaching focused on decisions, positioning and reactions. I would not simply tell the keeper to stand deeper. That can create other problems. Correct equipment supports the intended 9v9 experience, so this is an organisational matter as well as a coaching one. A practical point is that the decision should sit with the competition and club officials, not the goalkeeper or individual coach. Raise it before matchday where possible and ask what approved alternative is permitted. If the fixture proceeds, record the issue and coach the game sensitively. A repeated equipment problem should be escalated rather than normalised.