When our dog was very ill (he was a rescue dog who turned out to have bone cancer, and he became very ill very quickly after we got him) I took him to the vets knowing the vet would most likely recomend we have him put down at that point.
While I was out my Mum told my son that I'd taken the dog to live on a farm.
I came home in tears because I'd just held him while the vet gave the injection and he died in my arms.
And walked in to my son asking when we could go to the farm for a visit.
Firstly I had to find out what my mum thought she was playing at, and she said well you don't believe in heaven so what else should I have said? I said she shouldn't have said anything and waited for me to get back and speak to him.
I had to then take my son home and explain to him that grandma was wrong.
I said the dog had died, which meant that all the things that make someone alive didn't work any more, and so we had to make a grave for his body but that we could remember all the nice times from when he was alive that made him our dog.
DS wanted to know why my mum had lied to him and I said that she didn't want him to feel upset and so was trying to be kind, but I thought he should know the truth because we loved the dog and part of that love is feeling sad that we couldn't see him ever again because once someone has died they are gone.
We had a few tears, and I told DS that tears were important when someone dies, and so was talking about the person or dog who had died. So not to worry about upsetting anybody by talking about the dog, because we were all feeling sad anyway and talking makes that better not worse.
DS asked where the dog had gone and I said I didn't know. I said people believe lots of different things but we don't really know, but that while we remembered him he was still in our memories and that was a nice place to be.