To reduce the pressure on the day, give her a voucher and say it can be exchanged when the weather is suitable. It will give her something to look forward to.
I had a close look at the W&B on my flying club Cherokee 180C today in case the entire family come with me on the PPRUNE bash next week. I can take full fuel with two adults and two kids (combined weight 190kg) with about 40kg allowance to spare. So the folklore of 3 adults with full tanks, or four adults + tank to tabs is probably correct.
I would expect the south coast to France and back could be done on tabs with plenty of reserves. But dont forget to pack and allow for the weight of the liferaft and lifejackets.
W