Children are not born knowing how to tidy up a room. Don't laugh; that was a revelation for me when I only had littles!
I noticed something when I told my kids to tidy up: they would look around at all the mess and have no idea where to start. So I taught them.
Take them to their bedroom and plop yourself on their bed. Then tell them, "You put away all the books and you put away all the shoes." When they finish, they'll probably stand in front of you, silently asking: "What next?" So tell them.
"Now, you straighten up the books and you put the dirty clothes in the laundry basket." (The more kids in a room, the faster this goes. ;) )
Keep telling them what items to put away until the room is clean. Plan to sit on the bed and do the same thing every day for a week or two. Try to avoid a sarcastic tone that says: Good heavens, aren't you picking up on the pattern yet?
No, no, that won't help. (I'm ashamed to admit how I know this.)
After awhile of you training them to break down a room into parts, they are no longer overwhelmed by the huge mess, because they know how to tidy it up! Once you see that they are consistently doing it on their own (which you know because you're still supervising them), then move onto another room for new training.
Whatever you do, work heartily, as for the Lord and not for men, knowing that from the Lord you will receive the inheritance as your reward. You are serving the Lord Christ.Colossians 3:23-24