It's so cold outside for cats

I have three kittens and a mom cat I've been caring for for 6 months..Now it is so cold out and they insist on going under my deck -- it is enclosed with a picket fence, I put an outdoor heating pad and sleeping bag under there...I just hung a 60 watt ceramic heat bulb -- do you think this is warm enough for them to survive? Is it terribly uncomfortable for them when the temperature drops? They have a heated house I bought them on top of the deck (doesn't fit underneath) but refuse to use it....just worried that they are really cold...would like to bring them in but my Siamese wants to attack them...any advice would help....thanks..

Can you bring them in and keep them separate from your Siamese? Put them in a bedroom or bathroom.