Depends upon what you want to accomplish.
For an example, one idea might be guppies. They are colorful (therefore might be more interesting than other fish to a child) and they are prolific breeders, so your nephew will get to learn something as the fish produce fry. Simply start with something like three females and one male (the females need to significantly out number the males to "keep them at bay"). Within a month, you're likely to have to little guppy fry in the tank.
Mollies would be another example of a live-bearer, like guppies, that might be interesting to a child.
I would also suggest that before adding fish, you take the time to do a fishless cycle to make sure you're seeded filter pad has really taken root in the new tank. A good source of pure ammonia (without sufficants or soaps) would be Ace Hardware.
If you want something that is as simple as possible to take care of, Goldfish are obviously a good "hearty" choice.
I would also suggest that you consider leaving enough "room" in the tank that you could add a set of algae eaters later (i.e. don't max out the stocking of the tank). Once the tank is well established and mature, you could add some ottos to control algae, but they like to be in groups of three or more.
And of course to best make the tank as easy as possible to care for, you can try to keep the stock very low, perhaps as few as three fish in that 35 gallon tank.
Do you have any specific goals?
