How can I improve my writing skills when it comes to describing things, creating characters, etc. in order to be able to write better children books?

Start by finding things in your home that you can look at for at least thirty minutes. Once you find the item begin to think up words to describe it. Write down as many words as you can regarding this one item the repeat with another. Once you’ve done enough items to have a clear grasp on how to explain an item without saying what the item actually is, you move on to people.