The heaters you are describing sounds more like ones that will just keep a hole in the ice. It would not really heat the water enough for the fish.
At your temps I doubt the fish would make it.
I'm assuming your temps stay low for quiet some time and would freeze the pond over.

Bringing them indoors is an option. Lots of people in colder climates do this.
You'll need a large enough tub, pool, tank, etc...a filter, pump, and a heater if the place you intend to put them is not heated. Approx. size tank for 9 koi at 6 inches each....I'd give them at least 50-100 gallons of water per fish. So you're looking at between 450-900 gallons of water. Filtration is going to be your buddy here.

Do you have a garage or basement? Heated preferably, but if not you could purchase an aquarium heater large enough.
IF it were me going to bring them into a garage or basement.....I would build a tank.

Think of how a wall in a home is built.
Use 2x4s and make a box. Put plywood inside ; and outside would be great with some type of insulation between the wall. I'd use styro foam between the wall if possible.
You will then have to purchase pond liner and use some kind of underlayment(a styro foam sheet would be good on the bottom too if it's going to sit on concrete), and cover the inside of the box.

Not sure what type of filter you have but you might be able to bring it inside too.
I'm assuming that you shut down the pond for winter...no pump running.
If you can't bring it inside or don't want to then you can either buy another pump and filter or make a filter from a large bucket.....5 gallons or larger , depending on the size of tank you make.

Just remember....if you start with a new filter then your tank will have to fully cycle or you are going to have problems with ammonia building and could lose fish.
A new filter is going to take 6-8 weeks to cycle. And you do need a fish or two in there to get it cycled.

Now IF you have a big enough place indoors , that is heated, you could buy an INTEX brand swimming pool and put them in it. That is the only brand I know will not harm your fish. Most pools have an algae inhibitor in them and you DON"T want that.

No mater what you put them in make sure you put a tight net over it just in case they jump.

Oh, as for your fish having babies....I don't see how if they are only 6 inches long now. From what I know koi can't breed at that age.
Do you have other fish too?
Goldfish?

Ok, whew...sorry for the book. ;)
Ask all the questions you want to.
~Karen

Carp will survive in the bottom of a frozen lake all winter. I don't think the temperature would be your problem. I think oxygen and ammonia would be your problem. I did not loose goldfish when the pond was frequently frozen for 24 hours or so. Last winter it was frozen for two weeks and we lost them all. This winter I have koi and goldfish. I am going to keep a pump going which may require a heater.

Some discussion with links to serious wintering sites over at the Ponds forum. One interesting thread is overwintering koi.

I feel that your problem lies with those extreme temps, koi will hibernate. It drops to 0 F here for some time. Last year I left my filtration system run all winter long. Just the filters guts and used a surface deicer and my fish did just fine. The key is to make shore the pond doesn't completely freeze over. Keeping the filter running will oxygenate and at those temps algae doesn't grow. I just had to check on it once in a while virtually no maintenance.

Here is a link that might be useful: Winterizing Koi Ponds