So... the basics, you need to make sure that the pool is big enough for fish, as you don't want an overcrowded pond. You also need to get a filter for the pond, and possibly an air pump and some air stones for hot days c:
But I am not sure about how long the pool would last, since you need to have real sturdy and strong bottom on your pond to avoid leakage. If you'd seriously want a pond, I'd suggest starting from zero, and getting yourself a good base. You could buy either some specially designed cloth for this purpose, or a glassfiber bottom They sell them in many shapes and colors so you'd get the desired pond-look ;P But you could try out the pool first and see how it goes
You may also want to get some protective net or grid to put over the pond so no birds or cats will get your fish! I guess some thick, big plants around the sides would be nice too
Even if it's a pond, you'll need to do water changes and the like just like normal, but if you'll have a good filter there, you won't have to worry too much about that either. The water quality and temperature is harder to control in a pond, so you'd need to get hardy fish, like common goldfish, comets and shubunkins. I think fantails would be fine too, but no sensitive fancy varieties!

But about the feeding, you wouldn't need to feed any fish if you're just away for 2 days, especially NOT if they live in a pond d: There are always lots of bugs and the like that are out swimming for the fish to eat

Hope that helped a bit! I'm not a pro on ponds but... at least I shared what I know

You can build a pond out of almost any container.  Just make sure it is in partial shade.
My sister came to visit and she liked my barrel ponds so much she decided to make a pond herself.
She just went and bought a resin planter, added aquatic plants, waterlilies, and bog plants on a brick inside the pond.  They have fountain kits at any hardware type store that you can purchase quite cheaply and they are fine for turning any container into a fountain pond.  The kits come with a internal pump and a fountain head and hose that you will need.  I put 3 feeder goldfish into my barrel pond about a month ago and they were only 1/2 inch big or less.  They now are a whopping 2 inches!  I hardly feed them.  They eat algae that grows on the sides of the pond and mosquito larvae, bugs,  etc.  Ponds are fun!  It would be great fun to have a pond and your children would enjoy the learning experience.
There are some great pond books/watergarden/container pond books out there.
I do test my water in my ponds but if you add enough plants you won't really have to have a filter.
The plants will do that naturally.  I hardly ever have to do a water change.  I just add new clean water to the pond as it evaporates.

To make a homemade pond without a waterfall.

Dig a hole

Put a trash bag over it

Add any sinking accessories

And fill it with water

Trash bags may even break from the slightest pressure or because of sharp rocks. In other words, definitely not suitable for a pond in the long run.