The force on the hands of children and adults is always the same, but the force that children can bear is definitely much smaller than that that adults can bear. In the process of increasing the force between the two hands, we must first reach the maximum that the child can bear. When it increased, he could no longer bear it and instinctively "yielded".
Looking at it from another angle, arm wrestling is not about who has the greatest strength, but about who can bear the greatest strength. You can never break a pillar by hitting it in the wrong place, although it will never take the initiative to exert force on you.