I used to.
DH and I have tried relentlessly to ensure that his DCs weren't left out, and felt as much a part of the family as my DD (who is here 50% of the time) does. We holidayed together, had special traditions and routines, had responsibilities and jobs as part of a team, plans for the future, space and freedoms in our home.
It didn't work. Their mum openly contradicted DH in front of the DCs and professionals - in her opinion they are not part of DHs family and the DCs are too scared to contradict her.
My DHs DS recently completed a piece of schoolwork in which he was writing a recipe for "a happy family". Against the prompt Leave.... (such as leave to rise, leave to marinate, leave to cool) he had written Leave out the Man, he's done his job. DH saw it at a recent parents evening. The school photocopied it when DH asked for it 
Recently, I often said to DH that his DCs will always be a part of this family, but that his ExWs DCs can't be. His exW wont accept that the DCs can be both - so unless they choose to defy their Mum, we won't see his DCs again.
They're not my DSC anymore.