During the ceremony, unless the kids are a part of the wedding party, the parents are responsible for keeping their children still while you get wed. The reception is where it becomes a little bit more challenging.
If your mini guests are ages ten and under, getting them a separate kid height table is ideal. Depending on the theme of your wedding, provide arts and craft boxes for them to use. Stick with the colors of your reception so that it looks like you thought of them during the planning process, not to mention it will look amazing in the wedding photos. Put disposable cameras on the table so that they can take pictures of each other and whatever else they want so that you can get a different view of your wedding from their eyes. When putting your favors together, create something just for them. Cookies and milk are a great option.
As far as your menu goes, kids meals are cheaper as they will all prefer to eat chicken fingers over chicken vera cruz.
If you have lots of teens attending, you could create a cafe (coffee-free, but still) with tables and chairs, sodas, an espresso machine serving steamed milk, a variety of herbal teas and designer water, international newspapers and magazines, and a dozen vintage board games. Have music playing, too (cool music). Assign one member of your waitstaff to man the cafe. Your teenage guests won't want to leave.
If you definitely want the kids out of the way so that the adults can party carefree, set up a room just for the kids with a care giver, filled with fun activities for them to do. The younger ones, three and under, will be ready to sleep before the evening is over so having a designated nap area is also an awesome idea.
Whatever you decide to do, think of how you'd want your kids to be treated while at someone else's wedding. And if you can't accommodate them, not inviting them is always an option. Or not.