If they're twins, then the best option would have been to give them identical names and then it would simply be [NAME]s' birthday.
No need for all this unnecessary difficulty.
In a pub a few weeks back, a chap came in and didn't recognise the girl behind the bar, actually the landlord's niece.
"Who was that behind the bar?"
Sarah, Willie's daughter.
"Oh, how old is she now?"
Twenty.
"Oh, how many kids has he?"
Two, twins, a boy and a girl.
"Oh, how old is the lad?"
Everybody just looked at him for the fifteen seconds it took for him to realise why.