The real question should be - what kind of tree frogs are they?

66 degrees at night isn't cold for any tree frog, even tropical ones during winter months in the tropics.

If they are a tropical species I'd suggest a small heat pad (sold at any petstore) that sticks to the side of the tank, then cover part of the screen top with a towel or blanket during the colder months. If the tree-frogs hang around the heat pad source, cover more of the top to trap more heat until you notice them moving about the tank again.

A small space heater set up in the basement with the frogs might be another good source of heat. That's assuming it's a smaller room, or one with a low ceiling.