We had to take our poor old labrador to be put down when my DS and DD were about this age.
I didn't have the heart to tell them so I told them that that because the dog was very sick and old, the vet took her to a farm in the country where she can just sleep in the shade under the trees.
I know it was a cop out but I just couldn't bringmyself to tell them.
Last year we had a similar situation with our 16 year old retriver.
My DS gave her some food. Brushed her coat for about half an hour, cuddled her and asked me to take a photo of them together.
His DP had to do that as I was inside crying my heart out.
I couldn't even go to the vet with them.
I said in another thread that I was more emotionally attached to my DS than DD. (love them both equally though)
When my DGD asked where the dog was my DD said "The vet had to give her a needle because she was sick and now she is dead"
DGD 7 immediately said "Oh No poor DS, I et he's sad.
DGD then got the dogs collar and chain and put it them next to the photo.