I know people always say that they're leaving their (usually) wives and NOT their children - and the 'standard' is to tell them that Mummy and Daddy no longer love each other but rest assured they still both love the children very much and always will.
The fact is that, for whatever reason, they ARE leaving their children too, as a secondary by-product of their primary decision to no longer live with their ex-spouse. In most cases, they aren't abandoning or severing contact with them, but they ARE leaving them; if people reject the word 'leaving' then at the very least they are making them a much less significant part of their lives purely by virtue of the fact that they previously lived with them and saw them all the time outside of school and working hours, but now, days or even weeks regularly go by without seeing each other, and that becomes the new family 'norm'.
It's bad enough for a child to lose all of that time with their father (assuming no abuse or any need for him to have left aside from his no longer loving their mum), when they live with mum in house A and dad lives on his own in house B. At least that helps them to sort it in their minds a bit more, that they have their home with Mummy where they spend some (most) of their time and they also have their home with Daddy, where they also spend some (less) of their time. Either way, they are a main/important focus of both of their parents' lives.
However, when dad forms a new relationship and then has children with the new woman (and babies and little ones, by necessity, naturally get more attention than older children anyway), they don't understand the dynamics of adult relationships. All they will see is that "Daddy left Mummy and me and sees me every couple of weeks, but now he lives with the other lady and their new children together ALL of the time. No effort or arrangements are needed for them as the default is that they're all always together. Daddy lives with the new lady because he loves her more than Mummy, therefore Daddy must live with his new children all the time because he loves them more than me."
Yes, it's painful - relationship breakdowns always are; but it's very rare for the children not to suffer in some way. Everybody acknowledges the great heartbreak that children go through if a parent tragically dies and is taken from them, but many seem to gloss over the impact of one of their parents choosing to move away from them and then spending most of their time with a 'mummy replacement' and their new kids with her.