I deal with this all the time from the other side of things in my profession. However, today we had to euthanize our dog who was only 7 but had a congenital disorder that we knew from the get go would shorten his life. I have had to do this before. We actually had to euthanize our cat last fall due to liver cancer. At that time DS did very well and gave him a kiss then went back to playing. He seemed to understand without words (which he didn't really have at the time) what was going on and was very sweet to DH and me.
I have been away from home for 2 days at my very stressful new job. Last night DH called to tell me that Merlin was doing very poorly and that we needed to euthanize him today. I got home close to midnight. So, we didn't have much time to prep him which is what the article recommended.
Today we tried to prepare DS that Merlin would not be coming back with us because he was very sick. Of our 6 pets this one is probably the one DS was least attached to. He was very much my DH's dog and was so delicate that we were always careful when DS and he were around each other even though DS is unbelievably gentle with the pets most of the time.
We explained that Merlin had died and went to Heaven to be with Zig (our cat from last year and he did actually remeber I think). We wanted to be gentle but straight forward with DS. We really tried to avoid unclear statements like "put to sleep", "passed away", etc. DS is generally very intelligent and understands in most cases more than I expect him to. I read an article that basically said to explain everything to him even though he would not understand it. So, knowing DS who while sensitive is generally very steady and sure of himself and us, we went with that. He tends to make sure that he understands things. In fact when we had a brief talk about strangers, he went back to playing after talking then came back to me after a few minutes and said he wanted to talk with me (like I had with him) and reviewed all that we went over a couple of times to be sure he had it straight. So, he likes to make sure he understands things. Also, I don't want any fears of sleeping or going bye-bye to develop from this!
So, he responded pretty well. However, it has been extra difficult on us because throughout the day DS has asked us to repeat where Merlin is. He also asked about Merlin coming home and when we said he can't come home with us any more he would say "But he has to come home with us!" It is killing us thoiugh DS seems to be doing fine. I don't know if this will continue (if it was his kitten or our other dog or even the lizard I think it would have been much worse for him), but any suggestions would be much appreciated.
Also, I am repeatedly leaving for days at a time this month while we make the transistion to moving out of the area until DH and DS join me in our new location next month...So, though overall so far (you know 2 days) he has done fine with me being away, I am worried that this may change and I am worried about anxiety about the move developing.