OP, I'm going to be really honest with you. Because I just went through this a couple of months ago losing my 14 year old dog. And there is stuff I wish I'd known.
I have been with several dogs when PTS. My 6 year old staffy got a brain tumour and had to be PTS a few years back. He died on his feet - in a split second. I also stayed with a friend's 14 year old dog being PTS, same vet practice - and he definitely didn't die instantly. Maybe a matter of seconds but he was distressed. I never told my friend (they had to leave the room before the dog went - they couldn't cope with it so I stayed so it would have a familiar face with it at the end).
When my 14 year old went recently, she also seemed distressed. As the vet was doing it she remarked something about older dogs sometimes taking a bit longer because their circulations can be compromised. I wish she'd told me sooner. Later I researched online to try to make sense of what we saw - my dog also did something I have never seen before called 'agonal breathing'. A reflex makes them appear to continue to try to draw breaths after their brain is dead and heart has stopped (Sorry to be so graphic but I wish someone had warned me this could happen). It went on for a minute or more - even after the vet left us alone with her. It was very frightening and distressing to watch. I have been at home caring for my disabled son all the years I had this dog so she had been my life and soul, and only company in the day, day in day out for 14 years. There are no words for how much I loved my dog - and to see that was also beyond words. The vet said something breezy like "Oh that's just a reflex - don't worry!" Which wasn't much help but when I did look it up later I found out it was as she said, a reflex and if anything, a sign the dog has already gone. Sorry again to be so graphic.
I researched a bit further and found out you can have a pet sedated before the final injection (no-one has ever told me that and we weren't offered it But now I know the vet knew this could have happened and was indeed more likely with an old dog - I feel upset as we should have been offered this)
We couldn't afford to have her PTS at home which would have been ideal as she hated the vet's But I also looked this up afterwards (couldn't face researching it before - I was in shock about her having to go). But I learned that although it sounds ideal it is a bit like a home birth. Things can go wrong and if they do, there isn't the back up you'd get at the surgery. I also read it is statistically more likely an elderly dog will not die peacefully in its sleep and can suffer a lot in the last hours maybe in the middle of the night when you couldn't get to a vet - so I knew we did the right thing.
My husband made the appointment at about 10am and she was PTS at 5pm. It was in the summer holidays so the kids - including the adult kids and my youngest who had her their entire lives - were all at home from uni and school. We gathered them together and told them. They spent the day hugging her and saying goodbye.
The older kids were at the point they already knew she would have to go. It was obvious she had no quality of life.
Given the distress she had for maybe the last minute or two or her life and then that grisly agonal breathing I was so glad the kids - even the 21 year old - stayed home. I think they'd still be traumatised now if they'd seen it. It is probably also commoner in old dogs.
Love to you OP. My advice is under no circs let the kids be in the room when it happens (just in case) and give them time to say goodbye in their own way. Our staff we buried in the garden with full ceremony and grave goods (the kids each put a present in with him - his favourite toys or something of their's that was precious to them). Our bullie I was in such shock I couldn't face bringing her home and couldn't afford to have her ashes back. Simply had no money except what it cost to put her to sleep.
So we will never have her home. And I regret not just bringing her home now. I think having the other dog in the garden helped the kids to let go of him - in that they saw him off even if they weren't there when he died.
Love to you OP. Am here if you need me. Sorry for the content of this but I wish someone had told me these things may be rare I dunno but they DO happen.