My 8 year old son has Aspergers, so I know what you mean by how much they love their fish. Mine has 2 tanks in his room! How old is your son?
I had goldfish a long time ago, and thought the same thing that you did about just getting bowl and putting them in there. Now 3 aquariums later I have much more experience and I love my fish, but none are goldfish!
If I can make a suggestion, I would also return the goldfish. They are really big poopers and can mess up a tank quick
. BUT- Knowing how much Autistic children are routine-driven, that may be easier said than done. If you can't return the goldfish, make sure you test the water daily with an AP brand liquid test kit. It's not expensive and it's so easy. Just test every day, and if you see ammonia or a little later on, nitrites, rising, do a quick 20% water change. Eventually the water will be 0 ammonia, 0 nitrites, and low nitrates. Then your tank is cycled, and you'll only need to do a gravel vac and water change once a week or so. It takes me about 30 minutes to do 2 tanks.
I used these 2 links as a reference when I was new a cycling. They proved invaluable and helped me to learn about a nitrogen cycle with minimal effort.
Make sure you are using a water dechlorinator when you add your tap water. You can also check out BioSpira, which worked on all of my tanks. No ammonia overnight, cycled in 2 weeks. But only buy from a reputable fish store that keeps it refrigerated.
If you can return the goldfish, you may want to consider zebra danios and/or golden white clouds (which sort of look like mini goldfish). They are hearty little fish, don't need a heater, and can withstand the stress of cycling. The danios like to chase each other around the tank playfully, and it's really fun to watch them interacting, which is really good for our kids. The white clouds swim around also, but don't engage in as much play-chasing. The danios are also striped like a zebra so that is sort of neat too. In a 5 gallon, once the tank is cycled, you could easily have 6 in there and watch them school!
I also know how draining an Autistic child can be and I'm sure you are thinking the last thing you need is deal with testing water, and changing it out, etc. But it's not hard once you have it all figured out and start the routine. It's kind of like our kids! LOL
Feel free to send me a private message.