New players, and players with accounts that have been inactive a long time, have a multiplier for their rating gain and loss. Every game they play, the multiplier gets smaller, until they're gaining and losing a normal amount (about 10 points for a win or loss against an equally rated opponent).
This way, a new player or inactive player doesn't need to play many games before their rating is accurate.
In any case, new players are naturally below 1000, and also should ignore ratings. I'd say that at least the first 100 games are just about getting experience. If the child is upset about it, playing unrated games is a good idea.
If you or he is interested in knowing what his current chess.com rating would be, it's better that he play a variety of opponents near his skill level for the first 10 or 20 games (in other words no rematches at first). Maybe this will be useful info for you later.
I have introduced and encouraged my grandson to join chess.com as I believe the playing of the games will exercise his mind to think more logically.
I am however quite astounded at the punishing rating adjustments allotted to him in the two games that we have played and which he has lost as he is really only a beginner.
He joined and was credited with 1000 points and in the two games with me he has dropped to 921. I am currently at 1387 on my daily chess.
Now why I find this so disparaging is because when I play against an opponent with a much higher rating then I only lose a small number of points when losing but score handsomely when I win.
Yet my grandson lost 79 points over two games he played against me while I only gained 3 points in each game.
My grandson's high rating loss is quite discouraging. I have now challenged him to a new game with an "unrated" status so that he can first get a better understanding of the game before competing for points