Writing and submitting takes practice, and this business takes fortitude, because there is a ton of rejection involved. The good news is, if you really want a career as a writer/illustrator, you can take the rejections as inspiration to get better at your craft. Learning to revise is critical, and most of us don't really understand what "revise" means at the beginning of the journey. I sure didn't!
Others have made great suggestions. I would second that you write more manuscripts rather than focusing on getting an agent to sell this "one." This "one" is your baby. It's where you started, but an agent is going to want to make sure you are committed to a career, it's not about one book, it's about your voice in kid-lit, and what you want that to be. So maybe give that some thought.
Each manuscript you write will teach you something different about your craft and the process. There are so many things involved in building a story and it's HARD. Revising is not about editing words here and there, rather it's about noticing the pieces of a story and how they all come together as a whole. It's complicated!
Also, perhaps look at editor Cheryl Klein's book on revision: SECOND SIGHT. It's very good.