I dont think Meghan will come back to the UK, and if she did, I doubt if she would allow the Children to come to UK, and definitely not with Harry on his own.
Harry has made it clear he did not like being a 'working' royal, so I doubt he would take part in any official duties. And I do not think the public will want him to either.
It is possible he can return to a family role, and visit Charles, etc in the future, Balmoral, etc, but I think we should discount any visits of the children until they are 18, when presumably Harry could bring them over without Meghan's permission (here I am assuming that rules in the US are similar to the UK).
An earlier poster remarked that Harry did not seem to understand the Monarchy's role, or history, etc, but I think it is clear that from his book, 'Spare', he actually revelled at Eton in NOT wanting to know anything about the role of the monarch, or past monarchs.