Performance should not normally be the reason an U9 loses their opportunity to play.
There will be exceptions: a child may be hurt, overwhelmed, exhausted or need a short break. But “having a difficult game” is often exactly when they need the chance to keep playing, recover and experience the next moment.
England Football's 5v5 guidance describes equal game time as the priority and gives at least 50% as best practice. I would treat that as a floor, not a target for the children the coach trusts less.
I’d also be careful with language around taking someone off because they are struggling. The child can very easily hear “I made mistakes, so I do not get to play.”
At nine, matchday should still be a place to learn, not an audition to stay on the pitch.
