Okay, for the most part the writing world is done on word count these days since pages depend on font and format and everyone does that differently.
The average YA book these days is about 70-90 thousand words. Now I say average because of course there are always exceptions to the rules. For previously published authors they have a better chance at selling a longer book while debut authors usually need to fall somewhere in this. Anything under 50 thousand words is not a novel...well, YA novel anyway.
And because you felt the need to make the distinction I'm going to explain something else. Children's books cover everything from Picture books to Young Adult. They are organized into Picture books, Chapter Books, Middle Grade and Young adult. Young adult covers most of the teen years starting specifically about 15 years old.
First Book out for Representation.