Most unpublished children's manuscripts fail for the same handful of reasons, and almost none of them are about writing ability. They're about who the book is actually for — because the commonest fault in the genre is a book written for the adult holding it rather than the child listening.
The lesson problem
The single most frequent one. An adult decides a child should learn something — sharing, kindness, bedtime, confidence — and builds a story to deliver it.
Children detect this immediately and dislike it. A story that exists to instruct has a predetermined ending, which means nothing is at stake, and the character is a vehicle rather than a person.
The books children return to have a character who wants something badly. Whatever they learn is a by-product nobody announced.
The adult solves the problem
A child protagonist has a difficulty, and then a parent, teacher or kindly animal arrives and resolves it.
This is satisfying to write and flat to read, because the point of a children's story is a child doing something. Agency is close to the whole form. If your protagonist's situation is fixed by someone else, the reader has watched rather than participated.
Nostalgia instead of childhood
A great many manuscripts are about an adult's memory of being a child — gentle, hazy, sentimental, light on conflict.
Actual childhood is high stakes. Fairness, exclusion, fear, wanting something unreasonably badly. Books that work treat those as serious, because to the reader they are.
Too many words
Picture books are far shorter than most people assume, and the instinct is always to explain more.
The illustrations carry a large part of the story in a picture book, and text that describes what the picture already shows is dead weight. Writing for illustration means deliberately leaving things out, which is counterintuitive for anyone used to prose.
No reason to turn the page
A picture book is a sequence of page turns, and each one needs a small pull — a question, an interruption, an unfinished moment. A manuscript that reads as continuous prose has no rhythm when it's laid out, and that rhythm is most of what makes a read-aloud work.
Rhyme that doesn't hold
Rhyme is the most common thing debut authors reach for and the hardest thing to do well. Near-rhymes, inverted word order to force a rhyme, and metre that scans in your head but not in someone else's mouth are all extremely visible to an editor.
It also travels badly, which is why many publishers are cautious about rhyming debuts. If the rhyme isn't flawless, prose is a much stronger submission.
The test that catches most of it
Read it aloud to a child in the age band, and watch rather than listen. Where they look away, where they interrupt, whether they ask for it again.
That's considerably more informative than any adult's opinion of the manuscript, including your own.