Carson Daly's son, Jackson, is good friends with Christina Aguilera's son, Max.

'The Voice' USA host, who briefly dated the 'Your Body' hitmaker in 2002 and has been friends with her for 15 years, can't believe how much their lives have changed and says the set of the series is very family friendly.

The 40-year-old host told ''My son and Christina's son play together. We sit there and look at them playing and say, 'Can you believe this? Look at us now!'''

Carson's four-year-old son is expected to be among the guests at Max's sixth birthday bash in Los Angeles tomorrow (12.01.14).

The former MTV presenter, who also has a 16-month-old daughter Etta, with his food blogger fiancée Siri Pinter, claims his family are his biggest inspiration to work hard.

He said: ''After this many years, I know who I am. My life is not television. I'm a family man first and foremost. Everything I do in this business, it all pays off when I come home.''

He added: ''Our family time is like happy hour. Siri and Etta are cooking and I'm making kiddie cocktails with Jack -- that's our thing.''

The couple, who want as ''many healthy kids'' as possible, got engaged last June but are in no rush to get married.

Carson explained: ''Our lives have been crazy. We'll get married when we have time. The husband and wife thing is just the bow around something. Our end game is we want to be together forever.''