Hi hi. May I ask a question- is it if you've a child already, there's a rule which is not needed to do 过大礼?

Interesting question! Frankly speaking I have never heard a rule saying not need to do if you've a child already. But hey I may be wrong as different dialects may have different customs. I think the safest way is to consult the master that you are engaging when choosing the auspicious date. Nothing will go wrong with him haha.