I have poultry heat lamps on my rabbits. It has a sensor attached to them that makes them turn on at dark, and off in the morning. It works quite well. You will have to experiment to see what height the light should be at.
It's hard to answer without knowing what kind of enclosure they are in, but adding insulation to as many sides as possible should help a lot. And of course there should be a sloped roof to keep snow from piling up.
drafts/winds are the biggest danger, so make sure their hutch is sealed up enough to protect them from that. also, if you can buy a bale of straw (it's SUPER cheap), it's a bit better at insulating than the hay.
here's a great outdoor set-up that someone has posted on youtube:
Depending on where you live and how cold it gets insulation is always good plus I find that if you build a bit of snow up around the cage and the run it helps; kinda like an igloo! My boys seem to like it and it keeps the wind out. Unfortunately this is my first winter with them and I couldn't tell you how the melting is going to work until it happens to me!
I raise Holland Lops and they live outside all year so I think I could give you a few suggestions I don't know if you have your bunnies outdoors or inside a building but I have advice for either way.
If your buns are outdoors the biggest thing you need to worry about is keeping your buns dry and out of the wind. Put their hutches in a place that is bloced from the wind. To block the wind you can use a tarp or you can use this hutch cover:
To keep your bunnies warm put a large blanket over the top of your rabbit's hutch because heat rises and it will block the heat in. I would definatly stop using hay for bedding and use straw because that will provide your bunnies more warmth and insulation. I use this snuggle safe heat pad and it works amazing! You just microwave it and it stays heated for 10 hours and there are no cords to worry about
Currently I have all my buns in my garage because it was 30 degrees F and it rained and now it is below 0 F outside and everything is frozen and icy and I'm waiting a few days until everything thaws out to put my bunnies back outside
My bunnies do fine in the cold like most rabbits do because they have soo much fur so I'm sure your buns will be fine I hope my suggestions helped!!
Btw, I also have a 15 week old Holland Lop! Below is a picture of all my buns if your interested