When I write I always start with character. I decide who my protagonist is going to be and any interesting character traits. I can spend weeks just thinking about characters before I even write down a word. For me, character comes before plot.
I am very aware that in a children's book the protagonist needs to be a child. but, not any old child - it needs to be a child who is pro-active, brave and can use their initiative. But, it is not only in a children's book that the characters can't sit there and expect other people to sort out their problems. This is true of all fiction.
I have learnt over the years to trickle information about character into my writing rather than write blocks of character description. Andrew Melrose once said in a workshop at a SCBWI Writer's Day in Winchester, many years ago:
Make your characters whole, make them real, make them people. Leak the clues deliberately and at a good pace. Let them evolve.
I have always tried to follow this advice. However, I have also found that sometimes you need to state the obvious as it is not always obvious to the reader. When you get to know your characters it is easy to assume everyone else knows just as much as you do. This is not necessarily the case, as I have recently found out. So signposting your characters is very important.