Hard to say.. a bigger city will probably have awful air, I wouldn't want my baby breathing it. But, it will also have more work opportunities and access to foreign foods and goods if that's important to you.

Avoid the North-East, and the large cities of inner provinces, like Lanzhou in Gansu... Zhuhai should be not too bad, maybe Xiamen too. It's very urbanized. Suzhou used to be good, but now the smog never goes away. Small cities in inner provinces might be nice, if you pick wisely (ie. not steel mill or cement factory around), but then bye bye comfort ... and jobs. The problem is that the jobs is where most of pollution is.

Vancouver has a good Chinatown.
San fran, LA. Dont worry, you have options.

the coastal cities are not so polluted with the wind currents, and many big cities have the imported milk, very expensive though, bring a good supply with you, 2 adults 4 '23kg bags' one suitcase full of baby powder, clothes are cheap here but the jeans are terrible bring 2 or 3 good pair, they dont have any good ones in china, not too many, jeans are heavy.

I have been living in china for 7 years. My advice is don't go to china to settle down. Pollution, corruption, food quality, values of locals will make you crazy. If you don't have any chance choose Hong Kong.

Don't come to China, sorry but I wouldn't want my baby (if I had one) breathing toxic air, eating vegetables loaded with cadmium and other heavy metals, drinking bottled water that might come from a tap in the nearest back alley, being touched by farmers who didn't shower for a week (yes even in big cities they are everywhere), and so on.

Middle class and wealthy Chinese spend huge amounts of money to insulate their children from the pollution. If you want safe food for your baby you will have to buy imported stuff and it will be at least 30% more expensive than back home, because you can't trust the "organic" or "non-pollution" stuff in China there is no way to trace it, imported baby formula is extremely expensive since the scandals around local brands killing babies, there is big money to be made and people are willing to spend that much to protect their children, smugglers from HK and local sellers having baby formula know this so they charge +300% on it. 

You have been warned.