R.J.
We went "sleep wear appropriate" for years. I honestly can't remember WHEN exactly we switched to a comforter/duvet (we don't use flatsheets in our house...just bottom sheet and duvet/down comforter. The duvet gets tossed in the wash once a week with the fitted bottom sheet)
Sleepwear appropriate:
Summer (no air conditioning)... short sleeved pj top & Pullups
Autum/Spring (or air-con summer) ... Long sleeved/long legged knit PJs
Winter ... Polartec long sleeved long legged PJs or sweat suit
These are obviously approximate. If he could sleep stretched out (aka not curled up chilly or cold) and not be sweating, I figured he was dressed appropriately. We went with two pieces so that I could change his diaper easy in his sleep.
Nowadays we use the "four seasons quilt" down comforter from IKEA. For a twin I think it's 40 bucks. It's one thick down comforter and one thin one. You velcro/button them together for when it's very cold, just use the heavy one in spring/fall, and the light one in the summer.
(We started using the down comforter in the crib, but we kept my son in his crib until well after he turned 3. We just made sure the extra length of the twin comforter was pushed towards the bottom, so it wasn't like he had 4 comforters on, all piled on top of him.)
None of ue use topsheets...because a) hot feet run in the family, and we all like to have our feet sticking out b) tangling problems that happen if you're trying to get your feet out in your sleep with a tucked in flat sheet c) ease of making the bed in the morning (just loft, smooth, done). My mum claims I'm going to ruin the duvets...but the mine has been going 8 years strong. I have to figure, they're the same fabric as the sheets, no? It's not like I'm washing the whole comforter every week. :)