How much goalkeeper coaching should a team coach actually do?

You do not need to become a specialist goalkeeper coach. At U9, a team coach can give a goalkeeper a really useful foundation: be ready when the ball is live, get into a sensible starting position, communicate, make simple decisions about holding or releasing, and help them become comfortable using their feet. You can also create lots of game-like goalkeeper moments rather than separate technical drills you are not confident delivering. If a child becomes especially interested in goalkeeping, that is where some specialist support can add value. The important thing is not pretending. If you do not know the detail of a diving or handling technique, you do not have to invent it. Coach the football decisions you understand, keep the environment positive and help the keeper stay connected to the team.