When to have my first child?

I am planning to have my first child in year 2020/2021(32 or 33years old) and that is also the time when I have fulfilled/almost fulfilled the Minimum Occupational Period(MOP) for my HDB flat. I will then sublet my whole flat and move to my parents/parents in law's place to stay for a year. According to my girlfriend, her mother has already agreed to such an arrangement even though that will be in 9 years time.

Rental of a similar property in the area is around $3500. Assuming rental stays constant till 2021, I will be able to rent out my flat and receive a passive income of $3500 monthly. After taxes and expenses(additional allowance for parents in law), it will be approximately $2800/mth or $33,600/year.

Apart from the financial benefits of this plan, I'll also be able to spend more time with my child and my future parents in law could take care of their grandkid as well. As my HDB flat is quite a distance from my girlfriend's home, I could save tonnes of travelling time and costs if we stay at my future parents in law's place. Most importantly, my girlfriend is happy with this arrangement.