Have little guests roam in room of their own.
Antsy kids won't sit still for a minute, let alone an entire wedding reception. Give them space to play in a separate kids' room where they can gather after the meal -- with an adult or two there to supervise. That way, they can happily make all the noise they want. Fill the space with age-appropriate toys, games, and craft supplies. You might also consider hiring a performer, such as a clown, magician, or storyteller, to keep their attention.