There is no standard dinner prayer, but Xavier University offers several simple options which include "May all be fed. May all be healed. May all be loved." This simple prayer was written by John Robbins in a work entitled "The Food Revolution."
Other sites, like Daily Prayer and Lords Prayers Words, also offer a number of sample prayers that can be used at meal times. There are many families that simply speak a prayer over the meal as it comes to them instead of reading or quoting something that they memorized from another source. The traditions vary from family to family and can also vary between Christianity and other religions.