Well the kids are home so why not make the most of the time. I have 2 children of my own, teens they are now, so I understand the motivation part. Think of something that is near to their heart that will motivate them to get purging and cleaning up their bedroom and/or toy rooms.
If you are working and not at home this week, leave them a task a day, i.e. dresser drawers, purge and get rid of what you do not like or what does not fit. Anything that needs repaired or hemmed, have them put out where you can take a look at.
Next day, all their papers and books in their rooms. Next day toys, and so on.
If you are at home, you may be able to turn this around in a day or two.
Remind them if they have room in their closet and you see they only have 2 pairs of jeans you will want to buy them another pair, but if you can not see in their closet for stuff, why would you buy more?
Good luck and let me know how you make out. Why not do before and after pictures and send them to me, if I have any more tips I will send them through to you.
All the best and enjoy the sunshine.