Try to figure out what the author did right and how she managed to do it. Read the bad ones twice, too, maybe more. Sometimes it's easier to spot what's bad than it is to spot what's wrong. Find their mistakes, learn from them, and you'll be less likely to make the same ones in your own writing. If grammar is your Achilles Heel, work on that.
Good grammar is an essential part of good writing, and good writing is what gets published.
There are books on just about every aspect of writing that can point you in the right direction. Writing courses are easy to find. Never stop learning. You can never know it all, but it doesn't hurt to try, as long as you practice what you learn. In order to write for children you have to know them.