I agree with Toots...this excuse sounds flimsy and I wonder what the real issue is.
Clearly they trust her somewhat to watch the kids. Is it a driving issue?
Would Flora be open to hearing the real issue and dealing with it? Like if it was her driving, could she have another adult drive? Or does she have car seats? Or could the parents go with her?
OP here. I don't know if it's a driving issue. I know that Flora sometimes has the car seats when she looks after the kids, but she doesn't usually drive them anywhere as their dad drops them off and picks them up, and I suspect it's a hassle to swap the seats in and out of cars.
As for Flora being open to hearing any real issues, no, she wouldn't. She doesn't take criticism well. If there is a real issue, I doubt Son would bring it up unnecessarily as he doesn't like to start rows.
Son, DIL, and the kids have gone down to see Flora's father on occasion - with and without Flora. I think it's just their busy schedules that's the problem with them visiting more (as the trip there/back and the visit take up an entire day, especially when the youngest has such an early bedtime).
On the plus side, Flora was talking the other day about finding time to go visit her dad and didn't make any comment about taking her grandchildren or not being allowed to take them, so I'm hoping she's decided to let the idea drop.