An anxiolytic was given to a child by a NW FA a while back...he is now in jail.
As far as antihistamines are concerned, one should be very careful. They often interact with other drugs, such as antibiotics, and can have disastrous adverse reactions. In addition, the risk of inappropriate dosing, paradoxical reactions (child goes wild rather than sleepy) are good enoung reasons not to recommend their use in planes for the purpose of sedation.
I suggest a quick trip to the library were many books have tons of ideas on how to deal with children while travelling (plane, car or trains).
No need to abuse with drugs...