I have chickens and I have ducks. I will tell you that any lone duck will be at the bottom of the pecking order with your chickens. Any duck hen will prefer another duck to interact with.
You will enjoy duck eggs as much as chicken eggs, although you might not be accustomed to that. My teenagers will always choose a duck egg when making fried eggs or an omelette as they are larger. Our ducks eat the same feed as our chickens (with green supplements) and the eggs taste the same. Duck eggs are best for baking bread. I do not know why but I've proven it again and again. Ducks do not lay as reliably as most chicken hens.
Finally, ducks tend to be more pet-like, which will amuse your 2 year old. This is highly dependent upon the duck breed, but I believe Pekins would be best suited or muscovies. I have call ducks, rouens, and flying mallards.
Enjoy your duck choices. You're doing a great thing for your child by allowing him/her to experience livestock in a home setting.