The most adorable part of any wedding has to be the ring bearer and the flower girl. Even by themselves, these little homeys are capable of making the occasion well worth remembering, with one coming in just before the bride enters, while the other usually stands beside the groom holding the momentous 1.0 carat diamond ring that will seal the deal, so to speak.
Kids this age, while young enough to be extremely adorable, are also sufficiently mature to understand what they are supposed to do on the big day. Get more than one for each role, so they do not get intimidated walking into a group of silent adults. Let the ring bearer have a pillow made out of fake rings attached to the real one, so he does not lose the latter. Dress him up smartly with suspenders and conductor hat, which is the main wedding theme color.
Kids this age know they should behave, at least for the day. Bribe each according to your own discretion. Have the ring bearer carry the diamond engagement ring in a box he will protect as religiously as his favorite action figure, and dress him up in a dapper suit.