Not a historian, just someone who likes to lurk and learn new things, but I can maybe point you to a few similar questions I’ve seen about kids and their obsessions while you wait to hear from an expert.

This post has an ancient Roman source from u/rkiga that mentions a toddler obsessed with birds. Further down in the comments, u/Kyncaith briefly discusses Onfim, a young boy whose doodles on birch bark were preserved, and that these drawings show he daydreamed of becoming a warrior and drew himself as a wild (but friendly) beast. Link to the source here.

u/kingconani recently discussed how literature in the 19th century for young girls let them pretend to be princesses, still a common obsession as evidenced by Frozen mania.

Those are the only two I remember seeing off the top of my head, but hopefully it will tide you over until you get a more detailed answer from an expert.