We had a large, 30 gallon tank. It was beautiful and we enjoyed a variety of multi-colored community fish. It definitely took work though. Cleaning, etc. Balancing the pH level is what you do in the beginning, mostly. You get the tank first, set it up and get it stabilized, and then you add fish a week later. Kids find that frustrating, believe me, but you have to do it. Thereafter, you only deal with pH when you change the water (we did about 1/3 of the water every 6 weeks). We bought a simple and inexpensive test kit with 2 chemicals, and it was easy. Our son actually enjoyed watching us add the drops to the test tube and checking the color against the color guide in the enclosed booklet. So it's actually a lesson for kids in proper care of fish and ideally keeps them from adding stuff to the water, like their juice in case the fish are thirsty! If you don't do the cleaning, I'll warn you now, fish waste builds up (ammonia mostly) and the fish die off. We found we did the best when we bought fish from the pet store in our town - exact same water.
Someone gave us separate small bowls of Betta fish (one per bowl) and they did not last long and were, frankly, boring.
You can find small tanks on Craigslist - I'd suggest a 5 or 10 gallon tank, a light, a small heater, and a small filter to both clean out some impurities and oxygenate the water (vital), and then let the kids pick out the color gravel you want, 2 fake plants and 1 little item like a castle or sunken ship. Part of it is for decoration but some fish need little places to hide. Don't put the tank near a window - sunlight will cause algae build-up (notice in the pet store, they are far from the front windows).
Get a small net, a siphon for cleaning (you clean the bottom of the tank only where the fish waste sinks into the gravel), and a small test kit with a couple of test tubes and pH tester. Your pet store can tell you what you are most likely to need in your town - something to raise or lower the pH but probably not both. They will also test your water for free any time you go in, but do that only if it's convenient for you.
Be prepared that fish get stressed when you buy them, and they will die over time. Be prepared for whatever you plan to tell kids about death! Ask the pet store for instructions on how to acclimate the fish to the tank - you buy them in a plastic bag, then you float that entire bag in your tank for 20 minutes to be sure the water temp is the same and the fish aren't shocked. There's also a way to get the fish used to your tank water - let the store tell you, or PM me and I'll explain. This is something you only do when you purchase the fish - you don't have to do it again.
I think you want fresh water community fish. We enjoyed various kinds of tetras and mollies, as well as scissortails. I'd start with maybe 2 of the same, then the next month add 2 more. We also used a couple of different types over the years to eat the algae (there's always a little and they are nature's tank cleaners).
Fish are pretty low maintenance (despite the way this post sounds!) but you have to do it right to minimize the work long-term. What you'll probably find is that the kids are fascinated short term, and then the interest wears off, and you're left in charge of feeding and cleaning. They can be beautiful and peaceful to watch, and it's a nice alternative to TV though! And if the kids DO lose interest, that's your reason for not getting anything else like gerbils and guinea pigs and a dog! Those require much more work!
Let me know if you have any questions. We enjoyed all our fish for about 15 years. After a major New England storm in which we lost power for 5 days, we lost all the fish (no heat - the water chilled and that was the end of them).