Philippa, your post could have been from me 8 years ago when our youngest son and his family moved to the States. We were, like you, upset to lose close contact but DS said they wanted us to visit them as often as possible and for us to appreciate that this would be for longer (2 or 3 weeks) at a stretch whereas if they'd stayed in the UK, we used to see them maybe for weekends.
We were able ourselves to contrast their situation with our own nearly 50 years ago now, when we left the north of England to live in the south. No FT then, no Skype, no emails, we didn't even have a phone.
Now we FT, WhatsApp and email them but as others have said, having the next visit planned helps enormously.
Like Reader54, we're trying to help with his divorce situation now, it's messy, I want to wave a wand as mothers always do, lots of phone calls, FaceTime, messages, emails are helping.
Philippa, what helped us hugely was the very first visit when we became part of their life, taking the children to school, going grocery shopping, out for casual meals, out for coffee, to the park, etc. When we came back and then had phone calls, etc., we could visualise where they were. Plan your next visit and come on here and share your distress but remember (as I'm sure you will) that it's a fabulous opportunity for them - and for you to see a bit more of the world. Hugs.