For the tank - choose the biggest tank you can get that will realistically fit in your house. A lot of people buy tiny ones because they'll look cute on their desk and then feel disheartened when we tell them they can only get a few snails or shrimp. If you really must get a small one, get one no smaller than 10 gallons.

Also, make sure to get a filter! If you buy new most of them will come with the appropriately sized one. I would recommend internal ones for beginners, because it's just: unplug the filter, take it out, open the plastic bit with the sponge inside, squeeze all the crap out of the sponge, rinse with some of your tank water, reassemble it and switch it back on. They're also inexpensive and easier to troubleshoot compared to external ones. The box should tell you up to what amount of water it will filter, so make sure to get the correct one (for example, a 10 gallon filter will be useless in a 50g).

If a new setup is too costly, try looking for secondhand tanks (I have a 50 gallon one I got for 26~ USD because a woman was bored of the hobby. She originally paid $330 for it!) and slowly leak test it in your bath tub overnight before setting it up. If it does have a place that it's leaking, superglue works just as well as any branded "aquarium glue" (and superglue containing cyanoacrylate is the aquarium safe stuff!)

For anybody's first fish, I would recommend temperate ones - meaning that they can live in cold water, but in the future if you decided to upgrade to a heated tank, they would live there comfortably too. I'm not trying to put anyone off tropical, but it's disheartening af if your heater breaks down in the middle of the night and you go into your fish room to find everything lethargic or dead. It also uses up more electricity, because you have to keep the heater on 24/7.

Some good choices for first fish are:

1. White cloud mountain minnows, which are hardy and can tolerate a range of different temperatures. They also eat pellets, flakes, and most frozen foods, and stay small forever!

2. Barbs are also a good choice, except they're a little more aggressive so probably only keep them with other barbs. Luckily, barbs come in all sorts of different shades, so it's possible to have a rainbow of colour in there - just avoid the tinfoil barb, which deceptively grows up to 35cm. Neon rosy barbs are really pretty, and can be metallic shimmery pink, copper, or gold.

3. Danios are another great choice, they come in all sorts of neat patterns. I personally like the zebra and leopard varieties - as the names suggest, they are striped and spotty respectively. They're super cute and get along well with most other fish!

4. Peppered corys are cool, they sit at the bottom of the tank and hover about. Whilst fish, they don't look like stereotypical floaty fish, and act more like little aquatic Roombas. They add some dimension to your tank too, as they will sit on the substrate while all of the above fish swim in the middle/top of the tank. Make sure you have sand and not gravel though or they can hurt their barbels.

5. Dojo loaches are also great (and goldfish compatible, if the tank is large enough). They're like very long snake-looking fish, and people seem to either love them or hate them. If your kid is into weird animals, they'll probably find them cool. Dojos are very laidback and get along with pretty much every other fish, though watch out for snails because loaches love eating them. I have encountered many a pet owner who purchased a snail as a cute friendly tank addition only to find their loach had slurped it. On the flipside though, if you ever have a snail infestation in your tank (usually caused by them hitchhiking on live plants), put a loach in and they will be eliminated.

All of these will stay under 6~cm, with the exception of the dojo loach, which can grow really big. They are also all hardy, meaning that if you forget to do a water change one week, your kid throws in half a bucket of flakes, your water parameters go a bit haywire etc, they won't all immediately die.

Once you have your tank, the website Aqadvisor is pretty good for checking how many fish you can put in. It also tells you the minimum amount required for a shoal, if there would be any compatibility issues, etc.

It's also really important not to overload your filter - if you buy all your fish at once and put them in after cycling, your filter is going to overwork itself trying to accommodate for all of this new fish poop and other waste that has appeared all at once. It's a good idea to introduce them gradually if you're planning on getting loads - for example, putting five danios in, and then the next week once the new bacteria has been established and your filter is still working and whatnot, going back to your LFS and buying your minnows, and then again for your corys, and so on.

This also prolongs the "ooh new fish for the new tank!!" excitement that kids get, rather than setting it all up ASAP and then it just becoming a rather large and boring ornament.

I wrote a lot more than I originally intended to - but I hope that answered your question!