I've done a bit of research about how to calculate the number of fish per litre, and I know that most threads advise against such a small tank, but for a child, all she would want are maybe 5-6 very small fish - like the cold mountain minnows, danios or tetras, and a few shrimp maybe?
I don't want to get too many, or have to upgrade as I won't have the money or space to do so/
Hi It's too small for the fish you mentioned, but if you add a heater you could keep either a solo male Betta splendens or a shoal of small tropicals such as Ember Tetras or Celestial Pearl Danios with a few shrimps.
The tank will need fishless cycling which takes 6-8 weeks to do before you can add any fish so if you want fish in it by Christmas you'll need to get started pretty soon. This article explains why you have to cycle the tank and how to do it, but if you have any questions please ask: https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm
A heater will only cost a few pounds so not much more. The 1 inch per 4 litres "rule" is not very clear. It only applies to small, shallow bodied fish for one thing and doesn't work for many species. For example, you couldn't keep a 12-inch fish in 48 litres of water. Then there's always a minimum tank size to consider for any given species which varies with how active they are, how much waste they produce and how much territory they need. So you start with the minimum tank size for a specific fish, and then stock that to no more than 1 inch per 4 litres, but only for small fish. For zebra danios for instance, the minimum recommended tank is 3 feet long so about 100 litres because they need a lot of space to swim and need to be kept in quite big groups to prevent them being too aggressive with each other.