Okay, I know this isn't your most favorite rooms to get organized, but lets not procrastinate, lets get the hard places out of the way first! You will really feel like you have accomplished something if you do.
Lets start with the older children first. Have each of them go to their rooms and find 5 things that are in good condition that they are willing to give away to someone less fortunate then they are. Put these in a box to be taken to a homeless shelter, a women's shelter, a hospital, etc. Be sure it is a place where you children can go with you so they can see the smiles on the faces of the other children when they receive their wonderful gifts.
Chances are, the things that have been left out on the floor, the dresser tops, etc are the things that your children play with the most. So that is a given that those things will stay.
Take a look inside their toy boxes, what is down at the bottom of the toy box? Those will most likely be the items they seldom play with. Take a box and box up at least 1/3 of their toy box. You are going to label this with their name and the date and place it up in the attic or out in the garage. You might want to make an inventory of the contents as you do this. That way, if you have more than one box, you will know where to locate things. As time passes, they may ask about a certain item and you can go retrieve it easily. My rule of thumb is leave them boxed for at least 3 months. Then you can rotate out the boxes, putting the things into storage that they now have out and bring out one of the stored boxes. This helps them appreciate what they have and things tend to stay fresh and inviting to play with.
Now you can work with the remaining items and get them into the toy box and up on shelves easily. When you are ready to rotate out toys, from the stored boxes, be sure to take 5 items from the things that are presently out to be given away. You are starting a good pattern of teaching your children to share with others.
Go through their closets and check for clothes that are too small. If there isn't someone in the family to pass them down to, either take them to a resale store if they are in good condition and you can get a few dollars out of them, take them to the shelters for someone less fortunate, or give them away, perhaps through www.recycle.org. But whatever you do, don't just leave them sit in their closet if not being used.
Check their shoes, shirts, sweaters, jackets. Same thing, if too small and wearable, pass them down in the family or give them to someone who can use them. Otherwise pitch them.
Have the children help you straighten their bookshelves and go through any books that they feel they would like to give away.
They can help you empty the wastepaper basket, pick up any scraps of paper that are laying around (my grandson's are savers of gum wrappers for some reason) and get those picked up also.
Check the dresser drawers, are they old enough to fold underwear, and pair up socks? If so put them to work and let them help you.
Dust down the surfaces of dressers and desks.
Chances are you are going to have to break this into several 15 minute cleaning sessions, that's okay, just do 15 minutes at a time and go do something else in between time. You don't have to finish it all at once.
You will be surprised at how your children will enjoy having a neat and tidy room, be sure to give them lots of praise on how great it looks and how much you appreciated their help getting it tidy.
With some luck, the hard part is done for awhile. Plan on doing the "closet search" for outgrown clothes every 3 months when you rotate out their toys. Other than that, they should be able to go in and pick up their room every day in 15 minutes or less. Clothes put up, toys off the floor and where they belong and trash can emptied.
Be sure to let them know that this is an expected routine for them to do every day. To pick up their room. IF they don't, you can impose rules such as what is left out gets taken by a parent and put up. If you want it back, you will need to do additional chores to earn the item back. My daughter's have found that this works great with my grandchildren and they started this when they were 3 yrs old!