Whenever my mum or dad are talking to me they refer to each other as "mum/dad" but I think that if my kids are around they should use "nanny/grandad" (their choice - I don't care what they call themselves). Or at the very least say "your mum/dad" so the kids are clear who they're talking about. Eg if my mum is talking about my dad she'd say "Dad'll be home soon" and then my kids get confused thinking their dad is coming home.
Inevitably they often forget when speaking to their grandkids as well and will refer to themselves as "mummy/daddy".
It's not a huge deal but it annoys me so much every time they do it.