Apologies, my initial post was rather curt - I using my phone at the time.
A small tropical set up will not be that much more that a cold water set up.
Both need a good filter (air pump unnecessary - more for cosmetics), both need sand (better than gravel, kinder on the fishes mouths), some form of lighting is preferable and plants.
The only extra expense for going tropical is the heater and they are not pricey.
Then a small school of tetra, guppies, platys (only one type mind otherwise you are in danger of overstocking) and you are good to go.
Read up on the nitrogen cycle and how to do a fishless cycle - very very important (and actually quite interesting).
Where abouts are you. I have a 90l tank going (and possible a heater) which you are more than welcome to have in you can collect from Sheffield. 15kg of well washed play sand from Argos or the like and a reasonable but cheap filter + lighting and you are good to go.
Remember water changes - 30-40% of the tanks volume per week ideally and don't let a fish shop (or your daughter) talk you in to overstocking the tank - biggest mistake all beginners make (me included).
Fish are great first pets but they require a surprising amount of care.
Happy to advise further if you need anything.