I'm on your side OP. It sounds like it is far too late to stop them going and I would also feel very hurt and devastated. A relationship over Facetime is nothing like a F2F relationship for a young child. My mum managed a great relationship with my overseas DB's kids but only because she made the effort to go to his home and be their summer holiday childcare provider every year when they were little. Even with that effort though, the relationship still wasn't the same quality as the one my kids had with my mum, as we lived close together and saw her all the time, often dropping in spontaneously. So I can completely understand why, if you had in your mind a close easy relationship between you, the DC, DM and SD you are now in shock at what you all lose when they leave.
Have they purchased the new property ahead of selling up locally and moving? I'm surprised they've been able to keep it quiet if they are relocating in a week's time! Maybe that means the move will be staggered?
I also have to say, if they do fully relocate I very much doubt they will move back for you to support them. By the time they realise they need the support, the practicalities of moving will likely be beyond them.
Being so far away really takes the option for you to ever help them out of your hands. Is that maybe part of your mum's thinking, so she won't ever become a burden?
I hope you can have a sensible conversation with them to get reassurance they have thought past the dream to the realities, along the lines of how close are they to a GP, pharmacy and hospital; what is elderly care like where they are moving; and what is it like socially where they are moving to (in case one of them dies and they are alone there).
I know a lady who moved with her DH to the countryside in their early 70s. Beautiful cottage down little winding lane just outside a village, with some land and room for chickens etc. All was fine for a few months, but unexpectedly the guy got diagnosed with cancer which he battled for several years. Wife didn't like driving, and struggled with the hospital logistics and caring for him at home. They found it almost impossible to get cleaners or private carers to help them as they were in a remote location. Winters were especially lonely and depressing for them - their kids worked in jobs that only allowed time off in summer so the kids and GC only came to visit once a year or so in the fine weather. The kidswere helpless when their dad was unwell, as simply too far away to do anything except offer moral support. When her DH eventually died she felt marooned knowing there would be no one to help her if she got sick. She felt forced to relocate again to a nearby town. She never really got her chance to live the dream. It's a huge gamble at that age, to trade the support networks you've got in your local community for the chance to follow a dream hundreds of miles away.