Hello,
My friend is having a gravestone made for her daughter who died during birth

She has decided she want the following phrase on the gravestone:
Fell asleep in mummys arms
I think this is missing an apostrophe? The people making the gravestone have changed this to:
Fell asleep in mummys' arms
I might be wrong but I don't think that is right either? I think it should be:
Fell asleep in mummy's arms
My wife thinks mummys' arms is correct - something she remembers from school tells her that because it is someone's mummy the apostrophe should be at the end?
Please can someone help us with the correct grammar as obviously this is something we want to get right.
Thanks
James