And Fern acts as a mother to her in the mornings, and when she's shopping / scavenging. It's a bit mutual, at this point.
(Though, yeah, I'm fairly certain that Frieren's childish behaviour is almost entirely intentional on her part, and that she can turn it off and act like an adult when she wants to, which admittedly isn't very often; she might also be a bit feral from all the time she lived alone in the woods.)