This is traditionally a job for goldfish which seem to have gained a reputation for being easy (and disposable!) pets. Perhaps your parents replaced your childhood goldfish when they died to save the heartbreak? Either way, the truth is that these are big, messy fish that need a large aquarium and are far more work to keep properly than more modestly-sized tropical fish. Trying to keep a goldfish in a small un-filtered tank or bowl involves exposing it repeatedly to poor water quality that could easily be termed cruelty. Not many people know that goldfish can easily live for more than twenty years when properly kept.
So, packing away the soap box, what are the best options? Goldfish still have a place but plan ahead and get a large aquarium, perhaps even consider a pond for them to move into after a few months in a spacious tank. A far better idea are small, colourful tropical fish such as Platys (Xiphophorus maculatus) which are domesticated and come in a range of colours, all happy to mix and easy to breed.
Whatever you decide, manage expectations and don’t make the mistake of promising ‘Nemo’ in a tiny Peppa Pig aquarium! Us fish folk are a hard-hearted lot and won’t put the emotions of children above the lives of aquarium fish when asked to deliver the impossible. Key to success is patience and an understanding of the problems that come from a new aquarium that lacks the right conditions to keep fish healthy. New tank syndrome is one life-lesson that children don’t need to learn, let alone pet fish.