Not trying to attack you...but threads about tipping usually go south. As such, the DIS has put a ban in most tipping discussions, especially debates over how much to tip. If you look in the dining section of the DIS, there is a sticky thread there regarding tipping if you desire more information. I think there's also a tiny bit of info in the top of this thread.
That said, I'll try to answer your question on a very basic non-rule breaking level. Guests do have the option of tipping for turndown. Timing varies, and I haven't paid much attention, but sometimes we've been at our room when they have come by. I'd say usually it's between 4 to 7pm. Staff always knocks. Turndown is slightly more than just turning down the bed sheets. They also refresh towels, empty the trash.
One possible option, should you desire to have a little fun with in-room tipping (any reason), is to use a clearly marked envelope. (or use the hotel notepad for a quick note.) It's also practical. What makes it 'fun' is that you and/or children in your group- can personalize it with a picture. Picture drawing can be a great way to keep antsy kids busy. Sometimes, if you are very lucky, the staff member might 'communicate' back. Further, if you are going to be out of the room most of the day, envelopes have an added purpose/advantage - again should you desire to tip either room keeping or turndown- you can designate which staff is supposed to get which money. Hope that helps (and doesn't break any rules.) Phew, I think I added enough qualifiers!