for a younger cham I wouldn't let the temp drop lower than 65ish. maybe 60....
are you looking for an answer on how to keep it that warm? if so.. don't use a light emitting heat source. Use a ceramic heater if you need to raise the temp in the cage. If you can, it'd be better to get a space heater for the room the cham is in... this way you don't risk burning your chameleons.