Hi Sally, as the others have said, carry on with the one you love. I don't write children's fiction so could not comment on which one is the more commercially viable, but I think that we all start writing a story which we want to tell, with characters and situations which interest us.
Like you, two years ago I was in a dilemma, my first book had been rejected on two rounds of submissions and the second I was in a mess with. I joined Jericho (recommended by other writers via Twitter) and haven't looked back. My suggestion would be to watch all you can on the Harry Bingham courses, the old Festival videos where they have agent panels, published writers telling their story, etc. If you can't get a MS appraisal (I feel your pain, it took me ages to save for one) then try for a beta reader, I found a great one, which helped me enormously.
Go with your heart. It really does pay off. Eventually I drafted out book one again and sent it to a writer I'd met via NaNoWriMo and she helped me edit it. It's now going to be published in March 2022. So don't give up. However quickly or difficult to define as a genre (as mine is) all you need is a great story with characters and belief in yourself.
Jericho has loads of help. Most of it free if you're a member, and the people on here can help you more with feedback. Good luck.