From my understanding, there are some intra-family bars against negligence tort actions. The rational behind this is to stop fraud in insurance claims between family members and also so that the court won't disturb "family harmony." In most states, husband-wife intrafamily tort immunity has been abolished. I won't give you the full out exam answer about all the rational behind parent-child tort immunity, but generally parents are liable to their children if their liability stems from an action outside the scope of their parental duties. I can't remember the exact case, but the case in our caseboook that covered this issue had a similar fact pattern to your hypo. Applying the general rule, the court found that driving a automobile is not something in the scope of parental duty, therefore the child could sue his mother for injuries.