If you want a strong character in your story - and, of course, you do - you have to know a lot about him. Everybody comes with baggage, and so does your character (yes, even or possibly especially when you're writing about children). Unless your character was born during your story, he brings past experiences that will shape his behavior.
We have to love our characters for who they are. And we have to know who they were in order to find out who they become.
Start with the basics: age, height, hair color. Then add more obscure details: what's his favorite TV show, what's inside her purse, does he bite his nails, is she allergic to peanuts. The more detailed you get, the better you know your characters, and the more loudly they will speak in your story. Picture it like that first night with a new roommate at college when you sit up all night and talk about yourselves. You know each other so much better come dawn it's like you were lifelong friends.
Some writers spend days creating detailed histories for their characters. I don't have the patience for that, not when a story is itching to hit my computer screen. So I recommend spending a morning. Write out a full page on your character's history. Then set it aside. Don't - and I can't stress this enough - do not cut and paste from your character history into the story. Most of your hard work will not and in fact should not appear in your final piece. There's nothing that will stop someone from reading your story faster than a long exposition about every move the character made before arriving at work that morning. You need to know it, not the reader.
But it will be there. It will be there in the way he lines up his pencils before starting to work, or when she checks her teeth in the rearview mirror. Your characters will develop these quirks seemingly on their own because you know them so well. And they will start using their own voices. Just listen.