First there's the Quality Time Delusion. This is when the author confidently states, that she/he has read this story to her/his own children and they absolutely loved it. Now on the face of it, this sounds like a ringing endorsement from the target audience. So how could it be anything but a good idea?
Actually, what you're really saying when you make this assertion is that your child enjoyed the extra attention they got from being part of mummy's or daddy's project. They liked having their opinions taken seriously. They got caught up in their parent's dream about becoming an author and it excited them. The truth is that your child's enthusiasm is no guarantee of anything except that you spent some quality time with them and they liked it.
Next there's the Children Of All Ages Blunder. In this one the author glibly asserts that the story is intended for all children from the age of six to sixty, or some similarly hackneyed phrase.
Frankly, this is a stupid thing to say. A five year old lives in a different world to an eight year old, a ten year old lives in a different world to a thirteen year old. The idea that your story might work for all of them is an admission firstly that you don't know anything about the market for children's books, which is highly segmented, and secondly that you don't know much about children.
Finally there's the hoary old chestnut of the Friend Who's Done Some Illustrations to go with the story. Take it from me, unless your friend is an experienced professional illustrator of children's books, never include his or her drawings with your manuscript.
This is because even though authors see writing as an art, to publishers it's a business and, like every business, it involves an element of risk. An unknown author represents a risk to a publisher. An unknown author combined with an unknown illustrator doubles that risk.
So don't even include that art-work on the grounds that you think it will give the editor an idea of the kind of book you have in mind. The only idea it will give them is that they should put your manuscript on the rejection pile right away.