I was just trying to put Charlie down for a nap (he hasn't gone down for weeks, but I'm an optimist), and he said to me, "Don't ever put a cover on me, I don't like it!"
This has been going on for months, and so we dress him in thick pajamas... and when it's especially cold out, we'll sneak back into his room after he's asleep and put a cover on him.
Does your toddler (or school-aged kid) insist on sleeping without a cover?