I really should set up a macro for this answer!!! A goldfish is not a suitable fish for the aquariums that the vast majority of people have in their homes. They can grow to 60cm, produce a lot of waste and need a huge tank. A 17.5 litre tank is simply no where near big enough for a goldfish. My advice is to stop buying goldfish immediately. You are only going to cause more distress to your daughter as you increase the the size of your goldfish cemetery. They have probably all died due to poor water quality, in a tank that size you would need to be doing daily water changes.
I apologise if that seems harsh, but please look through the fish section of Answers and you tell me how many times you see questions on goldfish dying on here?
OK, to tackle your other questions. You need to make sure that you are cycling properly. Just putting dechlorinated water in a tank for a week doesn't make your tank cycled. The point of cycling a tank is to breed the bacteria that convert Ammonia, to Nitrite, to Nitrate. If there is no ammonia in the tank then you aren't going to get the bacteria required. You are then adding your goldfish who are poisoning their water very quickly, with no bacteria to make it safer for them. Personally, I use live plants and just add a little fish food, this then gets the process going and I find that after about 2 weeks I have cycled. You need to get a test kit and measure your levels yourself to make sure that you have gone through the full cycle. You are then in a position to add fish slowly as the bacteria colony grows to cope with the fish you add.
Water changes depend on what fish you have, but you should never completely change your water. This will remove all of the bacteria that you've bred. I aim for a 10-15% change every week, but you can probably get away with a 25% change every fortnight. Again though, it depends on the size of the tank and the fish kept in it, there are no definite rules.
Unfortuantely, fish keeping is not a straight forward, easy hobby. If that is what you want, you are much better off buying a cat or a hamster etc for your daughter. You really need to research the fish that you are interested in and decide if you are able to provide for them. There are many beautiful fish available that would fit in your tank, but please have a quick search on the internet before going back to the fish store and don't buy anymore goldfish!!!