Assuming you want to play Online Chess (as opposed to Live Chess) - and assuming your grandson is already a member ...
PLAY -> Online Chess. Start New Game (the green one under Play Chess - not the orange one).
At the drop-down ...against, select an opponent I choose and specify's your grandson's login name.
Finally, click on create new game.
How do I play a game against a select opponent. I want to play with my grandson specifically. How do I do it?