I'm so sorry. It is heartbreaking to lose a beloved pet.
I would tell them, allow them to say goodbye, and ask if they'd like to write a note or picture to bury with her (if you're burying the ashes?)
Our 17yo cat died 3 weeks ago, and our whole family looked after her on her last day. It was fortunate that everyone was here, as I think they would have missed out on a priceless experience. I didn't take her to be pts as she wasn't suffering and she hated going to the vet. We emphasized that this process is also an important part of pet parenting -helping them at the end.
My kids are early 20's now - last cat died when they were very young and at a very different stage, though they still know where her grave is.
We wrapped her in a favourite blanket, with treats, had a funeral in the back yard and dd20 wrote a letter to bury with her. We all go out to say hi, and it seems like she's visiting us - a nail or a clump of hair randomly appear, and we hear her throughout the house. It's amazing how much she was part of every facet of our lives. Three weeks on, they're now at the reminiscing stage.
This rainbow bridge cartoon practically undid me the other day, but is comforting.