I did it to my parents. I have lived abroad since I was 19. As far as I know, they were proud of my and my sister's achievements. They brought us up to live our own lives. My children, too, now live a long way away, although the young family who were in the USA for 5 years have now returned.
It seems normal to me.
Loads of Gransnetters have families distributed all over the globe.
When your son married a French girl, surely you were aware that their moving abroad could be on the books.
You don't mention if your husband is still with you.
It is an unfortunate coincidence that both your children are moving further away at the same time. Make definite arrangements to skype/facetime and use all possible means to keep in touch.
Use your newly won spare time to improve your French, as the DGD will be speaking more French now, and, hopefully, you will be able to visit them at some point.
To keep up their English, however, you ought to only ever speak English when you are talking to them, and supply them with books, comics etc by post. (check with the parents what to send)
HTH.