my child couldn't handle the polar fleece sleeping bags, the heat would condense, and she would end up drenched in sweat.

for cold nights, and she wouldn't sleep with any sheets or blankets on her either, this is what i used:

cotton long sleeve spencer,
woollen longies (over night nappy)
all cotton sleeveless sleeping bag (in autumn and spring), sleeved sleeping bag in winter (they were $12 at best n less and big w)
sheepskin rug to sleep on
woollen blanket on her mattress between matress and sheeting.
(all natural fibres that breathe)
glass is a major loss of heat, so i made sure windows were well covered with thick curtains, to keep heat in. Kept her cot/bed away from window for that reason too.

for a very young one, SOCKS on the hands, instead of mittens, worked for us.

timed heater for middle of the night