How I keep my post-4 year olds practicing
I use three things:
1. ChessMaster for practicing games - the variety of computer opponents are great for kids (of all ages!)
2. Chess Tactics for Beginners for practicing tactics
3. Both kids go to fun and friendly chess clubs and camps (Chess Odyssey in Portland, OR)
My son, who is now 8, has a coach who is _not_ his father but who is also a 'kid', Oregon Junior Champion Daniel Gay. This 'Kids Teaching Kids' approach has made the lessons fun, pressure free and a great value proposition!