@Roselyn-Jerome Hi! Chantal's right--you really do have really nice work! I could see it adapting to YA covers. (The older the reader, the more room for realism there seems to be.) If you have lettering skills, all the better. But as Chantal said, and the SVSLearn guys says constantly, book illustration is all about storytelling, storytelling, storytelling. So you will have to revamp your portfolio. Check out some of the past Critique Arenas, because they talk about this a lot there. There is a class/handout somewhere with a portfolio checklist. (Sorry, I can't remember where at the moment. Maybe someone else will!)
I can relate because I had a previous career doing realistic art, and it took me longer to transition than I thought it would. You almost have to unlearn some things. But if you can accept the amount of work it will take, your drawing and painting skills are definitely there. And the good news is that there's not just one style for kidlit.
While you're at it, check out SCBWI and start following kidlitartists on Twitter and Instagram. That too can help you to get an idea of what art directors are looking for.
Good luck!