I agree she should absolutely not take them out without boosters! I don't know why some older folks forget this with older kids! I would get her two to leave there. Maybe even put them in her car.
Even for 3 hours a week, however, I think she should be able to take them along for errands or to be shown off to the other grannie (is that where they went?) My mother used to teach driving, lol, so I totally trusted her to drive them around town. No big deal. She took them for ice cream, or to her sister's and mother'. Unless she's a bad driver, i wouldn't make this an issue. She loves the grandkids, and they seem to love her, so I wouldn't make waves in the relaionship thee. Over the boosters, however, yes.
Regarding the "unless she...bad driver...". I am not sure where I stand here. For one, she made a poor judgement call when she put BOTH of my kids booster-less in the car. Second, she didn't call (she knows I would have been able to take her call, we've talked about it). And, third, she has become incredibly forgetful...losing things, forgetting things, etc. So, maybe she isn't such a safe driver anymore .