Fish shops very rarely tell you about cycling the filter. All waiting five days will do is tell you if the tank leaks and if your filter and heater work. A lot of shops will say to add some bacteria from a bottle, but most people who have used them find they don't work - and the ones that do only speed things up a bit not do it instantly.
The term cycling comes from the nitrogen cycle:
Fish secrete ammonia. This is toxic to them, it burns their skin and gills making it hard for them to breathe. In a cycled filter, there are bacteria living in the filter which turn this ammonia into nitrite. But nitrite is also toxic; it binds to the fish's blood stopping it absorbing oxygen. In a cycled filter, there is another species of bacteria which turns the nitrite into nitrate. Nitrate is only toxic at very high leels, and we remove it with water changes.
But an uncycled filter does not have any bacteria in it so the fish will suffer firstly from ammonia poisoning, then once the bacteria start to grow and make nitrite, from nitrite poisoning.
Cycling a filter means growing these bacteria. It takes a couple of months for enough bacteria to grow in the filter to support a tankful of fish. The easiest way to do it is to add a source of ammonia to the tank and wait till the bacteria grow, then get fish. There is a 'how to' thread on the way to do this here:
http://www.thinkfish.co.uk/forum/index.php/topic,16.0.htmlIt is possible to cycle the filter with fish as the source of ammonia but it does take a lot of work keeping these fish alive. It can be done successfully if you start with only a few fish (I would suggest no more than 3 for a 60 litre tank) and are prepared to do daily water changes for a couple of months. Here's the 'how to' thread:
http://www.thinkfish.co.uk/forum/index.php/topic,17.0.htmlCan I suggest you read through those two threads and see what's involved with both methods. Then you'll have something to help you decide which to do.
I do realise that your children might not understand why there are no fish in the tank (fishless cycling) but the alternative could well be explaining why the fish have died if you rush to get a lot of fish and don't do the water changes.
The other thing to ask is - what is your tapwater like? If you look on your water company's website, somewhere there should be information on water quality and hardness. You need to know the pH and hardness of your tapwater. They will influence which fish will thrive in your tank and which will struggle.