I used several throw blankets when my daughter slept in her toddler bed--and one thrown like a bottom sheet so she could be insulated from below too). I was able to fold them into the foot of the mattress, and they were longer than toddler blankets. On really cold nights, I also laid a twin blanket over the bed (excess just loosely turned under the bed, not tucked in).
Double layering pj's is good too, like close fitting leggings under sweat pants. Fleecy socks. (You know they'll be warm when they go to bed looking like the Michelin Tire Man!!)
But, anytime the temperatures drop into the low 40's, we put the heat on. My husband is a light sleeper and it takes him a few weeks to get used to the noise. I always worry about my daughter breathing in cold air for 10 hours straight--cold air exposure in a small child's lungs and air passages can also lower body temp and strain the body's immunology/defenses.