When I married my wife, I already had an 8 year old son who had always lived with me alone, when my former wife left us, I maintained a policy of never allowing any female girlfriend to be introduced to my son, this ONLY changed when I knew that my wife to be was indeed my wife to be!
When we married, and she took up residence with us, I knew that the priorities had changed, as the correct and Godly order is as follows:
If we keep to this priority of relationship, we will eventually get all things right, my experience is that if we change the 'pecking order' things go wrong.
So from the start we all had to adjust to a new series of relationships.
I could no longer JUST consider my son and myself.
My wife had two additional men in her life to contend with.
My son was no longer able to demand my full attention and had to contend with a new 'mother' who now had rights to discipline him.
We had problems, my son resented a new 'mum', my wife was unsure how far she could go with discipline, but we worked through those and in time my son realised that my wife was more of a mother to him than his birth mum had been, and as we progressed we turned into a family unit, and now we truly are reaping the rewards.
I have only ever had to 'slap' my son once, (aged about 6 years) not hard, just in front of his best friend when he went too far, and I did it to humiliate him.
So physical discipline has never been an issue, and I would never allow anybody except me to use any physical force on my child, but if I were a woman and had a new husband, I would expect that SHOULD the cause ever demand physical discipline, the husband should confer with the mother BEFORE any discipline was administered.
The most important element is that the birth parent fully explains WHY the new spouse is important to them as a family member, and that the new spouse fully understands that it will take time for the child or children to accept them as a father/mother to them.
It is also important to explain that no matter what the circumstances, the now absent birth parent will ALWAYS be their birth parent, and that relationship should never be discouraged unless and until the child decides that they want to change it.
Sorry, got a bit long there!
Maybe I should write this up as a hub!