The speed is roughly proportional to the applied voltage, so running at a higher voltage, will make it go faster. An interesting thing to note is it works in both directions. If you were to set the voltage and gearing, so it does 25km/h, then go down a steep hill, it would still only go 25km/h, even if the hill was very steep. What would happen is the motor would start to act as a generator and charge the battery up, as the motor's open circuit voltage, would exceed that of the battery. A motor controller can do this deliberately, at lower speeds, to perform regenerative braking.
If the listing is right, that the motor is rated to 400W, the maximum current will be 400/24 = 16.7A. Torque is proportional to the current and the amount of torque required, goes up with the cube of speed, if I remember rightly. This is because wind resistance increases, at higher speeds. Suppose you set your peddle bicycle so it travels 5m, for every turn of the crank. If you peddle at 60rpm, you'll go 5*60*60 = 18km/h, which is fairly easy for most people, even over a long distance. Now set the gearing so it goes 10m, for every crank. Most people will struggle to peddle at 60rpm, with that gear ratio, over a considerable distance: 36km/h is a racing pace, over a long distance. Perhaps if you're quite fit, you'll do it, but it's much more than twice as hard, as only going 18km/h.
Another thing to bear in mind is the safety aspect. How good are the wheels, tyres and brakes? What's the maximum speed you trust them to perform safely, on a wet surface, or over gravel? How old is your nephew? If it's still a child, you'll probably want to restrict it to an even lower speed.
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