Rated, you are almost certainly correct about that flight being nowhere near close to performance limits, and in fact most domestic flights would be the same.
But the basic premis remains. In my mind i cant reconcile the carriage of extra fuel for mum and the kids except when it becomes payload limiting - surely when you are chock a block with pax then the risk is greater?
I suspect we are arguing the same side of the argument. I dont advocate min fuel, never have and never will, but I also dont advocate carting fuel around the countryside to mitigate something that will in all probability never happen.
I prefer to manage risk by using enroute alternates, and a whole bunch of other ways.
I guess my view is we as pilots need to use all the tools available to us to manage our flights and associated risks of which fuel is just one