Raising a child correctly is largely dependent on the parents and sometimes the area's effect is minimal, but at cities form their own cultures different from the rest of the country that reside in at large, children are raised with a different standard of living and grow up with perhaps less ability to function in the outside world.
No, cities are not worse to raise children in, because there are opportunities in cities that do not exist anywhere else. Cities have culture and entertainment. You can find a better violin teacher, or a better doctor if you need one. You have options that you don't have in the country.
While there may be more challenges to living and raising children in a city there are also tremendous opportunities that make it valuable. Many cities have an abundance of museums and cultural institutions that the suburbs just don't have. Children can still play in parks while being surrounded by a more diverse group of people then most towns. Children also gain independence when learning how to use public transit.
While each city is unique, I would not say that it is worse to raise children in most cities. Children are young and simple minded. They don't draw the connections and differences between city life and small town life. Essentially, they will not notice the difference in their younger years.