I am going to ask you to ignore everything that has been said already. If you are determined to get fish, it is possible as long as you are prepared to do a lot of work. By the sound of it, you will be prepared to do this for the sake of the child.
Set up the tank, wash the gravel, work out how much Prime you need for the amount of water in the tank, fill the tank with water and add the Prime. Turn on the filter and the heater.
Buy a liquid test kit, the type with bottles and test tubes.
Buy a bottle of Tetra Safe Start - but ignore the instructions on the bottle. Tetra Safe Start contains the bacteria you need to grow, but it doesn't do it instantly.
Buy a siphon tube to clean the tank.
Buy at least one bucket and don't use it for anything else besides fish.
Buy fish food - but get the type of food needed for the fish. *
Now buy fish.
Platies have been mentioned but they need a much bigger tank than 30 litres. Choosing fish is not a simple matter but I don't want to complicate things further for you so I will just say that the most suitable fish for this tank are either one single male betta (Siamese fighting fish) or half a dozen endlers. Males only as females have a lot of fry every month and there would soon be so many there would be no room left to swim. Not both - either a betta or endlers. Both of these are brightly coloured fish.
*I mentioned food higher up. If you get a betta, buy betta pellets; if you buy endlers, get flake food.
As soon as the fish are put in the tank, pour the whole bottle of Tetra Safe Start into the filter and turn it back on.
Every day, test the tank water for ammonia and nitrite.
If either ammonia or nitrite are more than zero, do a water change.
To do a water change, use the siphon to suck water out of the tank and into a bucket. When you have taken half the water out of the tank, throw the water away.
Boil a kettle of water.
Run some cold water into the bucket and use some of the boiled water to warm it. Feel how warm the water in the tank is with your hand and use enough boiled water to get the water in the bucket to feel the same.
Add the amount of Prime needed for the amount of water in the bucket and stir it in for a couple of minutes, then carefully pour the water from the bucket into the tank. You don't want to pour it in fast as that would push the fish round too much and dig a hole in the gravel.
Continue testing the water every day and doing a water change whenever there is a reading above zero. This is what I meant when I said would be hard work - all those water changes.
Feed the fish only once every two days. The less food, the less ammonia the fish will make. Ignore what the food says about the amount to feed. Because they make money from selling food they say to use more of it than you need so that you'll buy more. Fish need a lot less food than you'd think.
After a few weeks you will notice that you haven't needed to do a water change for a week. Now you can start doing weekly maintenance water changes.
Once you reach this stage, you will be a lot more confident about keeping fish. We then need to talk about how to keep the tank running smoothly. You have had to learn so much already that I will leave tank maintenance for now.
The testing and doing water changes are what will keep the fish alive. I know it's a lot of work, but if you want to have fish quickly for the child's sake, keeping the fish alive for him is also very important.
