I think it is strange that N&F aren't very visible with Joachim - however I do not get the urge some people have to blame this on either Alex or Marie. How about Joachim getting some blame - if there even is something to blame any one about?

Joachim must have known for a long time when he and Marie would be in London - because his kids are in school, he could have insisted he should go earlier so they could be with him; he could have arranged the holiday with Alex differently - he could have done LOTS of things to take the boys with him if he wanted them with him. He seemingly hasn't - also we don't know if they even wanted to go.