July is pretty diabolical for heat and humidity, but in September you do run the smallish but real risk that a typhoon will shut down the city completely for two to three days. This could include the airport if it's a Signal 10, so make sure you have comprehensive travel insurance that would pay for a hotel if your flight to HK got diverted to Manila or Taipei and you had to stay there for three days.
I didn't mind the July heat too much, but I am Australian to start with and lived in HK for years, so maybe I am just used to it. If you are a delicate little English flower who wilts as soon as it hits 15'C you won't like it at all. Every building, car and MTR carriage is air-conditioned, so unless you're walking around outside a lot (which you probably won't be with a little baby), it's perfectly possible to spend a fortnight there in July wishing you'd brought a cardi because the air con is so cold.
Unless you are mad keen hikers (HK has stunning mountains and thousands of miles of walking trails), there's no real reason not to go in July.
Tin Hau is really convenient for everything. Taxis are very cheap so if you have a car seat that can clip in and out of a car (in this case a Toyota Crown, if you need to ring the manufacturer), it's worth taking with you.
All restaurants in HK are baby-friendly. It's very much part of the culture to integrate even the smallest children into socialisation. Only bars, nightclubs and the very poshest of Michelin-starred restaurants are adults-only.