I don't think it's selfish. However I don't know the individual circumstances either. In general I feel the suicide is a deeply personal action. Every person is different with different limits and different feelings. When leaving a child or children behind is involved, it does add a lot more complexity to the decision. I know someone who has put off ctb because they had a child. But they also adopted many unhealthy coping mechanisms to deal with their almost constant wanting of ctb. And tbh, kids aren't dumb, they picked up on those things.
So in closing, it may be selfish and it may not be selfish. I don't know what each person's breaking point is.