People should never hold an infant in their arms while riding in a vehicle. The baby will not likely stay supported under these circumstances and the result could be disastrous. Infants and children under sixty pounds or under 48 inches tall should use special restraining seats designed for their size. Adult lap and shoulder restraints are for children over 48 inches. If the shoulder harness does not fit without hitting the neck then one needs to use a child restraint system.
Children follow adult leads.
Use your seat restraints properly and be sure your younger passengers are stowed safely when you drive. The extra effort would seem insignificant if there were an accident.