One child seems left out even though the football is fine

I think the coach has some responsibility here, even if the child has never complained. You cannot manufacture friendships, but you can stop the same social pattern repeating every session. Mix partners deliberately sometimes. Use small groups that change. Give the player jobs with different teammates. Notice whether they are always last into a pair and quietly make the organisation easier for them. I would not make them the centre of a “let’s include this player” conversation. That can feel worse. Belonging matters because it affects how willing a child is to ask for the ball, make mistakes and keep coming back. The football being fine does not mean the environment is fine. Small coaching decisions can create more chances for relationships to form without turning friendship into another task the child is being judged on.