Denise7125 I am so sorry you are feeling devastated by this news. I would feel similarly, and I am not sure it is helpful for someone to tell you to "calm down". Australia is a long way away and obviously there are expenses involved in travelling there on a regular basis.
There is a chance they will change their minds, and some people do emigrate but decide to return for a number of reasons, one being missing family.
When they say you can also go out there, I am not sure it is that simple. Have they really looked into that carefully? I believe UK State Pension in Australia is frozen. However, if you can move there, would that be something you would consider?
If it finally comes to it, of course you will be upset but, as other gransnetters have said, you can still keep in regular touch, and either they or you can arrange visits now and then. Try and keep a brave face because it must be a hard decision for them too.