Staying warm when sleeping

Our house doesn't have a very good heating system and my daughter's room is always freezing no matter how high we have the thermostat. We really don't want to get space heater for safety issues, but we need to find some way to keep her warm as it's getting down into the negatives outside at night. Any suggestions for safe sleeping options to keep her warm? She's very independent so she won't sleep in our bed. Thanks in advance.