My DS and his 6 (!!!) kids live in the UK and me, I live in the S of France. He is getting divorced and has the 2 youngest GC living with him. He is going through custody etc. I am stuck here due to dogs, but mainly succession proceedings. So I can't be there for him, except virtually. He was still married when my DH pased on, but came over for a month to look after me. I feel bad I can't do the same, now with his problems, but I can't. Like you we do lots of WhatsApping. It's not the same, and doesn't replace a cuddle, or holding DGC in your arms. That is sure.
We have plans for next year when all should be over, beaurocracy wise.
It is hard to be far away, but if they are happy in Australia they are making a new life there. Try to encourage them, as it is an opportunity for all of them.
I get sad too, when I here maternal GM say she doesn't want our DGD to stay (she is slightly hyper) or DiL who abandonned the kids. If only...
But I can't be there. I live here since the 80', and DS moved back in the 90's. We are very close even though.
When you go back to see your DD and family why don't you hire a camper? Cheaper than a flat and you could profit over a school holiday, and travel with them maybe. I think it would be my choice... although each to his own.
Can DD and GC at least come turn about, to see you and her family? Even if you need to help her with fares, it would be easier maybe.
I hope you get into a state where you can be happy for them, and arrange visits more easily.
True that nothing can replace physical presence. But 20 years ago there was no face to face, only phone or write, so we are lucky to have at least that, I think.