I remember my first childhood holiday down south, when I was about eight - I couldn't believe my ears on the beach when I heard big children, many even older than I was (!), actually still calling their mums and dads 'Mummy' and 'Daddy' rather than'Mum' and 'Dad'. It sounded so hilariously babyish to me, and 'posh' too. Why didn't they speak normal (Yorkshire), like everyone else I knew - weren't they embarrassed at how silly it made them look?
Heaven knows what I would have thought if they had been using their parents first names!
Amusingly, my own older two DGC (9 and nearly 11) still often call their parents Mummy/Daddy, though it's gradually changing. I've noticed, however, that my youngest DGC (nearly 4), their cousin, has already started using Mum/Dad sometimes. Not their first names, though - yet ?