A boiled sweet - the individually wrapped hard sweets that they hand out on planes, especially before descent, the sucking is meant to alleviate pressure in the ears. But believe me I fly once , sometimes twice monthly and children can have the most terrible time. Ninety nine per cent of the time they are fine, but it only takes one child on a plane with ear pressure problems to make a racket that at the time sees interminable, the crew are used to this and given the time they will walk/carry the child about intermittently, if only to give the poor parent a break.