Needing a little advice please.

My new partner of two months sometimes looks after her sisters two young children who are adopted, but here lies the problem I have.

She offers to pick them up and take them out for the day etc. but whenever she drives on the motorway with the kids in the car she always speeds by exceeding the speed limit, quite often doing literally 100MPH on a 70MPH road.

I have been in the car with her when she has reached these speeds and I am not entirely happy with her doing these speeds but my problem is that I haven't said to her that it kind of bothers me.

Do you think she is selfish breaking the speed limit especially with two young children in the car and also myself?

Should I say to her that I'm not comfortable with it?