Is the idea to keep the pipes from freezing or to warm up the floor?

Yes

Here's the problem:

You are looking at a electric resistance heater. No matter what type you would choose, they all have the same efficiency which is almost 100%. The bad news is that electric resistance heat is the most expensive form of heat. To be clear, regardless of what type of portable electric heater you get, they produce the exact same amount of heat for any given wattage.

In addition, if you are talking a portable heater, they can be dangerous Anything with a glowing coil is a really bad idea. Most plug-in heaters draw significant amounts of current and the wiring is best if it's a dedicated circuit and new wiring. Don't plug a portable heater into old wiring. Too often the cords are undersized and get warm after long use also.

A better solution is to insulate the perimeter better and then open the space to basement (if that's possible) so that whatever heat is available to the basement gets to this area too. That also reduces the possibility that a failed heater that is "out of sight, out of mind" will cause the pipes to freeze.

I would insulate it with 1-2" of spray foam on the inside perimeter of the foundation, and be sure to seal the plate and do the joist pockets also. There's almost no good way to do that and seal the air also, without using spray foam. If you have used fiberglass, it's almost worthless in that application.

If, once properly insulated, you must have a heater, a permanent hard-wired convection (no fan) heater is the way to go. And again, no glowing coils.

Thank you, Jake. This is helpful.
In the bathroom that I’m concerned about, we have hard wired oil filled electric baseboards. I wonder if I just keep those on at night when the temperature dips very low (last night it was 11, which is extremely rare for us), if that will be enough.

Along with your insulating ideas. I think the crawl space is actually pretty well insulated. I’m just not sure if there’s a part where the pipe goes near the exterior (brick) wall where I can’t see and receives an extra beating temperature wise. Visually, the pipes disappear into insulation in the ceiling of the crawl space beneath the bathroom.

If the main concern is a water feezing in the crawl space then I suggest you instal an electic water pipe heat cable. These are made to keep water pipes from freezing durning cold weather. The cable has a thermostat which will turn the cable on when the temperature drops.

Here is a link to a inexpensive cable sold at Home Depot. There is a video showing how to install it with an inulation wrap.

Frost King 6 ft. Electric Water Pipe Heat Cable

I agree with the heat tape on the pipes. That's why I asked the question at the beginning if this was to keep the pipes warm or the floor warm.

Is the perimeter brick wall insulated? And if so what is it insulated with?

Sorry I missed that question! Right, to keep the pipes warm.

The perimeter brick wall is insulated. I assume it’s with fiberglass, because that’s what I see in the ceiling of the crawl space. I also see foam board and spray foam in the crevices.

The water pipes leading to the bathroom are the red and blue plastic ones, does that make a difference for heat tape?

Also, is there any chance that the pope freezing is the metal portion coming up out of the bathroom floor and connecting to the cold water supply? That part of the bathroom is further from the radiator and against an exterior north facing wall. Would it help to insulate that as well, even though it might be an eye sore?

PEX (red and blue plastic) piping is rated up to 180 °F. The pipe wrap will not achieve that temperature. I have read adhesives may interact with PEX piping. Don't use any adhesive and you should be good. You call always call the pipe cable manufacturer if you are concerned.

The pipe in the bathroom is unlikely to freeze as long as the bathroom air temperatuer is above 50 degrees.