I have been asked this question on occasion, but the correct answer stumped me. It is not listed anywhere in law books as far as I can tell.
Accordingly, I read a few articles and postings on the internet about the issue and concluded that there are no pat answers.
The best approach seems to be to remain flexible and just let the situation play itself out, while not directing the child one way or another.
I have read about situations where the child will refer to step mom as “mummy.”
Or, I have read about situations where the child will call the step dad tom-dad, or some other endearing mixture of the step father’s first name.
One thing seems clear: in blended families post-divorce, the situation is fluid and the terms that are used are not as important as the love that is shared. Children will always refer to their loving natural parents as mom and dad, and that is as it should be in my opinion. The closer the relationship with step-parents becomes, the more likely terms of endearment will flow freely and naturally, and be accepted by all involved.
At least, IMHO.
This material should definitely not be construed as legal advice for any particular fact situation, but is intended for general informational purposes only. For advice specific to any individual situation, an experienced attorney should be contacted.