I'll keep it brief as on my -notsofarking-- smart phone.
A few weeks ago my DD3 went to a birthday party at a school friends house. It was a cinema and pizza party. We were to drop them off and pick them up 5 hours later. The family are lovely and I knew she (and the ithers) would have a great time.
There were about 15 DC in total going - school friends and cousins.
A few days after the event I found out from a friend that the DC had being transported to the cinema in quite an unsafe manner. 
8 DC in a Galaxy. Ffs.
DF and I discussed this and decided not to address the issue with the mom (driver of this car) but just ensure that out DDs could never be put in this situation again.
This morning one of the parents of another invitee (went in a different car) came over to me and tried to persuade me to "have a go" at the mom. 
AIBU to let it lie and use the experience to firm strategies for the future rather than being confrontational?