I’m sorry your spirits have taken a pummeling. Rejection stinks, especially when it seems to undermine the whole project! Is there truth in the agent’s critique? Could be. “It’s not substantial enough” is a common critique because it’s a common pitfall as writers learn to distinguish a nice story for kids from a story that can sustain 32 satisfying pages and an $18 tag. Parents shelling out for books want reread-ability and to extend the conversation beyond the pages, and for that the stories need extra oomph from universal themes presented in a deep and/or fresh way, necessary and well-timed page turns, and lots of visual opportunities for illustrations. But then, maybe your ms is simply a fun romp, with fun language and energy that’ll entice re-reading for the sheer joy of it. There’s an audience for that. This agent feels your ms isn’t substantial enough? Fine. Your book isn’t for her. If you feel you’ve got the oomph or the romp, keep trying.