Well, for starters, we consider the way children engage with the world in general. They play, respond to outer stimuli, explore through the 5 senses, and imagine. It's no great stretch to think that children relate to God in the same way, or that God might make himself known to the children using these same things.
One of our volunteers in children's church told me a story about her just turned 4-year-old daughter. The little girl had recently heard the story where Jesus implores the disciples to let the children come to him without hindrances. A week or so later, she gathered up all of her marbles and put them in a small basket. She then spilled them out onto the sofa, loudly said, "Let the children come!", and gently picked up each marble and returned it to the basket. Over the course of a week, she repeated this "game" several times.
Though we have no way of knowing exactly what was going through this little girl's head, the story about the children coming had obviously caught her attention and she intentionally explored it further through the medium of play. And she didn't need any special, expensive toys either. Just a few simple marbles . . .