My then 11 year old grandson and I made a quilt for his soon to be born nephew. I planned about 40" square. He said, "No. We need to make it bigger so he can use it longer!" So we made it bigger - well, mostly a lot longer.
Now, I listen to his voice in my head when I make a small childs quilt, and try to make them about 50" square at least. I do use the lightweight fleece that you can buy at Walmart as batting, though, because it makes the quilt feel both softer and lighter when dragged around by a little one.