There's a huge range of electronic keyboards and pianos, so there's no short answer. At the very least make sure that the keys themselves are full width, even if there are less of them than an acoustic piano -- anything else is pointless. Yes, you can get two pedals on an electronic instrument although many will only come with a sustain pedal.
If the person is going to do grades, then the exams will be on a piano, which may feel very different from an electronic instrument. That depends on the electronic instrument, although it's fair to say that the more realistic it feels, the more expensive it will be.
For a six year old, I would say start with something cheap and cheerful. There's plenty of time to consider something more upmarket if they show an aptitude and a liking for piano. There's nothing to stop you mixing formal and informal lessons. Get some of the basics formally, so that they learn correct fingering and some basic reading skills. Play by ear too, and watch some of the fun (and free) youtube tutorials.
My own opinion is that it's hard to beat a real piano for keyboard feel and quality of sound, but there are practicalities like space to put it and the proximity of neighbours. Also, they are expensive unless you know how to recognise a bargain (which will almost always be from a private sale, not a dealer), avoid buying an untunable wreck which will bring you nothing but heartache and woodworm, and know how to tune it yourself.