Hi, first can I ask what state are ya from. Let your DH and the kids know that koi will weather very nicely outside. You want to stop feeding them when water temp. Hits 55 degrees. Koi go into hibernation. Their body metaboli slows down and if you feed them with water temps less than 55, the food can rot in their stomach. Is there a bottom drain in your pond, and what is your pond set up. Just curious if you plan to run the pond all winter or plan to shut it down

Thanks, we are in Southern CT. We have a bottom drain, a skimmer box, a bio filter, UV filter, etc. We chose the best of everything so that the fish would be healthy and happy. So far, so good.

The kids and DH know about the hibernation, but I think they are anthropomorphosizing and saying "hey, why would I WANT to stop eating and stay in the cold all winter, when I COULD be inside, warm and fed". KWIM?

We would not keep them in situ and feed them ... we would move them indoors. But, as I said, that would take an enormous tank (s) which then has to be kept clean, warm, etc. For 17 fish, year after year as they grow!

Our koi guy says the pond gets shut down for the winter.

Well, there is a few arguments here, I could see your family's point. I live in NE Pa. and the 3-4 months without my koi pond is rough. However, moving the fish will put a strain on them. Also, studies say koi that get to hibernate have better health and color. Its a way for them to rest.
Another point is, you would need a huge tank to keep them in. A small tank will put stress on the fish.
Reason I asked what your system is, I have two ponds. I have a 2000 gallon pond with gravel bottom. This pond I pull the pump, and set up a floating heater along with a submergible pump that circulates the water. As far as filtration, that is shut down for the winter. The pond is 2 feet deep and (knock on wood) I have not lost a fish yet over winter.
My 20,000 gallon pond, I pull the UV and run the bottom drains. The pond is not heated but air for the drains are pumped from a warm place. I may end up heating this pond eventually.
My point is, if your family wants to go through all that trouble, think about heating your pond. I would shut down the waterfall, this is a big heat lose area. The resources you would use for such a large tank. ( remember koi grow). With a heater, you and your koi may be happier.
There is a temperature range in which koi are more susceptible to disease. A heater will keep the temp out of that range. Do a google search for Winter koi pond covers

Thanks, Craig, for the advice. Sounds like there may be a middle ground to consider.

your koi will winter over very well in your pond just do two things,as stated above,stop feeding them when water temps drop to 55,they will have little or no interest in feeding.and make sure you are going to have a few warm days before you start feeding again.second make sure you leave a hole open in the ice.or the fish will use up all the oxygen in the water and die.i use a small aquarium air pump,to keep the pond from freezing over.

I know how hard it is especially the first year when you over winter fish. You think...ooo I hope they make it. Ive done it from the get go, and you can be pretty certain they will survive the winter especially with your pond that deep.
I shut my pond down in the winter too, no falls, just a heater and I put a aquarium bubbler underneath it, or buy it, just for back up. I highly recommend the Laguna Power Heat 500 watt de icer by Laguna and cover a bubbler control up with a little ziplock plastic container to keep the wet off in the winter so it doesnt shut down.
The heater I use has worked great for me. Cant remember who I got it from, but google it.
As far as if your beautiful fish are healthy and fattened up enough before winter comes, they will find a nice warm pocket on the bottom of the pond and rest.

Good Luck!
crazy4koifish

Thanks, Crazy, you seem to get my point. I know HOW you are supposed to over winter the fish, but it tugs at my heart strings to do it, and even moreso for my family. And since my DH thinks the safest --- and kindest --- thing to do is bring them into the indoor pool area (not the pool itself!), I am left feeling like any fish popsicles will be my fault!

I know I used to worry every winter when we would get a bad snow storm, even an icy storm, Id be out there checking making sure the heater was ok and th bubbler was ok...it is unnerving.
But hey, if your family would really enjoy them inside for the winter, try it the first year. Just make sure you do it before water gets too cool, cuz it will stress them to much and they could get sick. Anytime moving fish is stressful on them too, I hate to say, and you want to keep that in mind if your doing this back and forth every year, know what I mean?
Just be carefull and good luck!

crazy4koifish