Have the child, or any person, hold the racket up in the position where the racket face would strike the ball with the face directed at the intended placement point ( as if it was an actual serve). Now have them "ratchet" their wrist back and forth while adjusting the grip, slightly, one way or another, until they achieve the greatest freedom of motion from their wrist. That grip should be the one they use for all (or mostly all) of their serves.
Now, even though wrist movement should never be forced during the serve, there should be a freedom of motion, or free-play of the wrist, for a smoother service action- for greater power, control, variety, and less chance of injury.
For some people the grip may vary one way or another, but for the majority of people, it approximates the continental grip.