As long as there’s a floor drain reasonably nearby, emptying it would be a perfect application for a siphon.
I used to go to a back country ski lodge that used a big galvanized grain bin with a submerged wood stove as a hot tub. Hot water is hot water.
As for getting cold I don’t think it is going to make much of a difference, it is only in the past 25 years or so that tubs have been acrylic or fiberglass, tubs older than that are either enameled steel or cast iron. I am sure that most heat is lost through the uncovered top anyway.
Again though, you won’t get the last few mm or so of water on the bottom as the siphon will start sucking air and lose it’s prime. I’ve been draining aquariums for decades with siphons and it always ends up this way. While not a big deal for a fish tank it would be kind of nasty leaving a gallon of soap-scummy bath water to evaporate on the bottom of your tub… or a pain to have to climb in and soak it up with a towel.
Keep looking for the old house. You might also search for one with a pit dug out in the basement floor where you can lower a basket on a rope down to the kids to “rub the lotion on its skin”.
Once you’re down to the last half-inch or so where the siphon doesn’t work, you just lift up the tub on one side and tip it over to get the last of it out.
Yes that’s one way to get the last of the water out but it pretty much always results in that last little bit spilling all over the floor. Big stock tanks have rounded rims making any water poured out of them fan out wide enough that it’s really hard to direct it down a small drain… also you don’t just pick up a 6 foot long tank and hold it over the drain hole without considerable effort. You awkwardly tip it over and end up with a big puddle on your floor.
Worse yet, some of the plastic tanks have molded/folded top rims configured to allow over flow water to drain out about 4 inches below the top so the very top doesn’t actually over flow. These rims end up trapping water when tipped on their sides (that’s not how they were designed to be emptied) and are next to impossible to drain completely without a towel or a lot of time spent picking the whole thing up and tilting & shaking it out; the enjoyment the kids get will be overcome by the frustration of the parent cleaning up a tank not meant to be fully drained the same way an actual tub is.
Another old-timer here recalling baths in the round galvanized tubs that also served as rinse tubs for the old-style wringer washing machines. They weren’t as deep but they weren’t really comfortable either.
The only time we used a cattle trough for a tub was to go “swimming” in the summer time. The water was solar-heated! You wanted to skim off the top layer of cow spittle and straw and dead bugs first, but at least it was, um, wet.