I suggest encouraging them to do what I call "non-intrusive" activities. Meaning, instead of blowing bubbles on people's faces, they can play with their Gameboy, color a book, put together some legos or do anything that won't annoy others in the line.
Activity books keep the little ones busy while they're waiting as well, my children loved connecting dots and discovering images at that age.
It is just polite to think of others who may not be in the mood to participate in a family's loud or intrusive efforts to distract their kids... By this I do not mean to be rude at all, I guess I am sharing experiences in lines I wished were handled differently by some parents.