I rent a car for two days. I think a child seat is around $10 per day, plus tax, so it might be $30-$40 for two days. I feel like this way I don't have to bring my own. Do you know if there's a pitfall here ?

I rented a car seat for 22 dollars a day. It might be different for each car rental company. It's more convenient to buy a car seat directly from the rental company if you rent for a short time.

Melbourne, two days is not necessary to bring, very heavy, children's seats are more expensive to rent in Sydney

This price can buy several simple height increasing cushions online. The cushion is also not heavy, and it is also counted as an additional quota for luggage check-in at the airport. It is usually placed in the hotel and does not need to be carried around. After use, it can be thrown away without regret if not taken back to the country. In any case, I think it is more cost-effective than that of the car rental agency. Whether to spend more money or take more items is up to you to decide.