I've always wondered if, when Grand Duke Paul married Olga, he was forced to leave his children from his first marriage with Grand Duke Sergei and GD Ella, or did he leave them voluntarily? It strikes me as terribly sad and it's not surprising that both Maria and Dmitri had "issues" later on.
I've read that he was sure he would have to leave Dmitri behind for his military training, but that he was hoping the Tsar would let him keep Maria when he had finally settled. Before Paul and Olga were married, they had been travelling around Europe, staying in hotel rooms with their son Vladimir, for a considerable period of time (two years I think), as Paul still somewhat hoped that his nephew would allow the match. Since Paul was a widower, had already lived a dynastic marriage and produced offspring from it, both him and his mistress hoped for years that their situation would be regarded as different from other unions of the sort, but this permission never came. When Paul finally realised this, he decided to go ahead with the ceremony, even though he knew the consequences.
One can actually understand his decision. Maria and Dmitri, as much as he loved them, belonged to the crown, but his youngest son was his own and the Russian court had no rights over him, so maybe he thought it would be better to solve his and his mother's situation than to sit around, waiting for a third party to make that decision for him.
But even though he had to leave them, he never stopped writing and at least Maria always spent two weeks in Paris with him and his new family in Paris during the summer.
