Grandparenting

I do feel for you.
This is a tough choice.
I had the same some years ago but I stayed put.
I still feel pain for the missed years but I know I did the right thing.
So what can you do now? If your DH is happy will he still move back to the coast?
You could plan to return in a couple of years.
Then the new additions will enjoy visits to the seaside and you can be grannie by the sea ( that was me)
In fact my family have recently moved to be nearby. In fact the pain of the lost years hasn't disppeared yet. It is not easy at all is it?
My family have been here for five weeks and you have been near yours for six weeks.
Well it's not a very long time.
Could your feeling change?
IF not I would plan to return to the coast on the future. I do understand how hard the decision is.

I'm sorry you've found that the move doesn't suit you.

I agree, you could move back to the coast when the 'additions' are a bit bigger. Use the time in between to build a relationship with them so that when you do move away again they will look forward to visiting you.

In the meantime, can you afford a static caravan at the coast so that you can retreat there from time to time? Or at least take some holidays on the coast and maybe get some new ideas about where you'd like to go when you do move back there.

Looking after grandchildren should be a pleasure and not a chore. When it starts becoming a chore, you should give it up. You had 2 children of your own, so there must have been some maternal feelings there.
I shared childcare (with in laws) of my 2 older grandchildren (teenagers now) from the time , my daughter went back to work, after maternity leave. The only time I was really stressed was when my little granddaughter (3years old at that time) had a tummy bug with projectile vomiting and cried continuously for her mum. So, I had to ring my daughter, who promptly came back from work.
The fact remains that the older we are, the more tired we get and less able to do more than a couple of hours of childcare. That's where I'm at now. My sons have very young children and I have made it very clear, and they understand, that childcare will be limited to a couple of hours of babysitting, when needed. That's it. I'm older now, than when my daughter's children were born.
Talk to your son and daughter and lay your cards on the table.
Good luck !