Structure.  It is a word that gets bounced around a bit and we rather avoid. It holds slightly negative connotations and is easily misinterpreted to imply something too strict, too regimented.

If we change the word 'structured' to 'organised' we get a clearer picture.

Children like a sense of order in the physical setting. This physical order is the key to how they can choose and carry out their own individual activities in a peaceful atmosphere.