Does it constitute time-stealing from government time/pay while on duty transporting a minor, using cell phone for personal reasons and holding the phone in the hand?
Does it also mean that the Social Worker is violating the vehicle code and endangering the minor's safety by carelessly violating the the code?
Construction / Development Lawyer
Potentially, the answer to all your questions could be yes.
Was anyone harmed by the conduct?
The police would probably not be interested in any claim about driving while using a cell phone, unless they saw it themselves.