Yeah during the mage exam she demonstrated her childishness by... Taking control of the group, and teaching and protecting the two actual children that were on her team.

Or when she ran into the child Himmel in the forest, the writer showed what a child she was by... Maturely helping him find his way home and keeping him in good spirit with a magic trick.

But she does like to sleep in! That's undeniable! And no adult in history has ever liked sleeping in.