Lionel Richie's grandchildren call him ''Pop Pop''.

The 68-year-old singer's eldest daughter Nicole Richie has two kids, seven-year-old Sparrow and nine-year-old Harlow, with her Good Charlotte rocker husband Joel Madden - and they had no idea their grandfather was famous until they saw him perform live for the first time a few years ago.

He said: ''I'm 'Pop Pop' to [my grandkids].

''About five years ago, they said, 'We didn't know Pop Pop could sing!' That's the humbling point of being with the family. All they were talking about was, 'Oh my God, Pop Pop, that was amazing!'

''Keeping their attention at this age is pretty darn good. If you can do that, you're really making some headway.''

The 'All Night Long' hitmaker is gearing up for his Las Vegas residency, 'Lionel Richie - All the Hits' this November and can't wait to perform for his family and the crowd of all ages.

Lionel - who also has 18-year-old daughter Sofia and 23-year-old son Miles - says he once played Sin City and an elderly lady came up on stage and told him how ''sexy'' he was.

He told PEOPLE magazine: ''It's the microcosm of the entire world.

''The whole world's in the audience. What I love is they talk back to you.

''I remember there was this one lady in the front row.

''She had to be 80 years old and she's standing right next to the stage pressed up against the stage and had this glaring, staring look on her face.

''I gave her the microphone and she said, 'I just want to tell you how sexy you are.' Of course, the crowd was laughing and I'm blushing!''