Some children, especially toddlers and preschoolers, are overwhelmed by large groups so keep the birthday celebration small. Some use the rule of one guest for each year of your child's life, plus one. When determining the starting time for the younger set, keep in mind nap times. For older children the starting time will be determined as to whether you'll be serving a full meal or just cake and ice cream. Be sure to tell the parents of those attending when the party will end so that you're not left with children without parents after the party is over.
Simple parties are best.
Don't overplan the activities, but do have a few fun games in mind. For the younger children a sing-along, puppet show, or a pony ride are good activities. Another fun indoor game is 'Knock Your Socks Off' where the children crawl around on the floor and try to remove each other's socks. The last one with a sock on wins. For school age children, how about a dress-up relay game, putting on funny oversized clothing, running to the other side of the room, then taking the clothes off.