This probably happens more than you think for family and friends. My brother and I once flew in the cockpit from CDG to LHR when we were younger (about 7 &10 yrs old). My uncle was the Captain, and he gave us a quick lesson on take off and landing. I still remember the stares from other PAX when the FA came to get us, I guess we were one of the privileged few. Nothing wrong, if the kids are well behaved, just wonder if access is as easy today as it was then with all the new rules.