Upsetting why do people put items in a coffin with a deceased person?

The Ancient Egyptians did that, it was something to travel to the afterlife with.

It has been the custom to put grave goods in a coffin for centuries. However, it might be better to place personal items and a photo in a corner of the house somewhere, as a tribute.

I once took on a 14 year old dog whose owner had died. His family were not prepared to give him a home. He lived,happily we think,for another two years.
I cut off some of his tail hair to go with her original dad who had loved him so.

40 years is not such a long time when you have buried a child

I’ve never thought of doing this myself, though I know Catholics are sometimes (often?) buried with their rosary. I’m Catholic and I think I would like that. My husband isn’t but I’d like him to have something, perhaps a photo.
I’m really sorry this is still a cause of distress to you chocpud, I don’t think it was your MIL’s place.

I have my beautiful Springer Spaniel's ashes, she died 23 years ago. When I die they will be put in my coffin.

I agree with what Baggs suggested.

Did you ever have any bereavement counselling? If these memories are intrusive and upsetting, please ask your GP to refer you. Perhaps you could contact The Good Grief Trust and ask for help. Linda Magistrate set up TGGT as a charity to gather all kinds of support and I'm sure she would arrange something for you.

www.thegoodgrieftrust.org

It must be devastating to lose your child. I hope you can find peace.