Is there a basis to sue?

Yes. Although he was responding to a service call, there were violations of procedure and a failure to maintain concentration (writing an address down while traveling 120 with no sirens/lights).

Had he been following all procedures with lights/sirens and concentrating on driving and the wreck occurred then there wouldn't have been a basis, it'd have been an unfortunate occurrence.

I would say there is basis to sue. The officer demonstrated gross negligence in his job, causing the death of a 2 year old and severe injuries to the other occupants. The family could sue him, the police department, and/or the county. The department and county could be held accountable for improperly training and/or supervising the rookie officer.