Yes, the child antics have no bearing on the plot, but the point is to show that it's a light-hearted place where Schierke feels at peace, BUT her character is conflicted with wanting to help Guts complete his journey, as well as find out what grave taboo her teacher committed in the past. They're spending a whole chapter on shenanigans to make you FEEL the calm that Schierke feels, so that there's actual weight to her decision when she (probably) decides to leave this place in order to help Guts.
And all the people complaining about how Berserk isn't dark enough, not enough violence and disturbing stuff, that's what Skull Knight's character here represents. He has bearing on Guts' story, both in the plot and as a meta-commentary on Berserk itself changing as a story. Miura could've just made the story be nothing but the Black Swordsman arc, just a man getting revenge, but he decided instead to evolve the story and move it forwards. And part of that is Guts deciding NOT to be a rampaging monster who cares about nothing but revenge, which is what skull knight represents. Someone who forsaked any friends, family, relationships, and even his former lover, putting revenge above everything else, even his own humanity.
Previously, the Conviction arc was about Guts growing past his black swordsman character, because his decision to forsake Casca for the sake of revenge almost killed her. And when he rescued her from certain death, he decided he was going to put protecting her above trying to klll Griffith. And then, trying to protect her alone became unbearable, so he accepted the help of his travelling companions, which is right now in the story being tested with Guts seeing skull night's story. Will Guts go the path of skull night, and throw away everything for the sake of revenge? Or will he try to tread a different path?