Child abuse and neglect rates would plummet.
Due to supportive home environments, more children would grow into successful, highly functional adults.
How would secondary parents be regulated?
How would the public outcry be handled?
Where would the excess of children removed from unlicensed parents go?
What party would determine the requirements to receive a parenting license?
I acknowledge the gross right violations this implies.
It is more a thought experiment than a plan of action.
A baby has never come with a manual/instruction book, parenting has existed long before governments and laws, there is no one way agreed-ed apon correct way to raise a child (I doubt a government will come up with one).
The human race is still here after all those generations of unlicensed parents, all a licence scheme would do would provide another revenue stream for governments to exploit, assuming they could cover the cost of such a program in the first place.
Its a waist of resources and time, targeting those who will really need the money for the next 18+ years and no politician who would like to get elected would ever put forward such a policy.