Hi, and welcome to the forums!
To answer your question, some bettas cannot live with any kind of tankmates. I'd avoid the Endler's Livebearers in any event. If you are new to Bettas, you may want to go with just the betta for a while, as there is a learning curve no matter how much research you do.
Bettas can be interactive: they "dance" when they want food, and play games, and so on, but NOT every fish will do this. For a 3 year old, you need to consider how you can keep the tank close enough to where the child can see it but far enough out of reach so that the child cannot get their hands into it.
You'll need to get an adjustable heater, silk plants, and such for the betta. Here in the states, all the stuff you need can easily run you about $100 which is probably more like 60 pounds sterling.
Shere Khan -- Dwarf Gourami Akela II, Raksha, Phao, Phaona II, Sura II, Rama II, and Mowgli II -- Three Line Cory Cats Adnarel -- Golden Mystery Snail Kaa -- Zebra Nerite Snail The Nephilim -- Malaysian Trumpet Snails
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