At U9 I would usually let them play without telling them.
Knowing a scout is watching can change a child’s behaviour very quickly. They may force actions, stop passing or become frightened of making a mistake.
The scout is there to observe the player in their normal environment, so normal behaviour is useful for everybody.
If there is a specific reason the club or family needs to tell the child, keep it very low-key and avoid turning the game into a test.
I would certainly not announce it to the whole team.
Children will have plenty of time later to learn about selection and observation. At nine, protecting a natural match experience is usually more valuable than making them feel they have to perform for somebody on the side.
