I and my FRC team have been a running after school and summer Lego activities for over ten years now. We frequently change it up but the central precepts remain the same:
1) It must be self-paced. Kids come into these things with wildly different experience and appitudes. If you try to be a "sage on the stage" and lecture you'll bore half of them and loose others. Our video tutorials at
www.STEMcentric.com are key to making this work. Students have to watch (with headphones) and do the the exercises under "essentials" before they can do anything else.
2) There must be a "magnetic field" to which they can align. In other words there needs to be a reason why they want to get more and more done. For FLL, that's provided by FIRST. For other times we might pay them in RoboBucks for things they do and have an auction at the end, we might have an "Invention Convention" at the end where they show off what they did to parents, maybe it's just candy bars. Sometimes we'll build a giant Rube Goldberg machine with Lego robots, sometimes they are Bluetooth controlled capture the flag robots, sometimes we encourage everyone to make different projects. Competitions of one type or another help provide that organizing field.
3) Don't exceed two students per robot/computer. We have a 1:1 ratio now. Otherwise there will always be someone with nothing to do. You don't want them finding things to do!
4) We don't go younger than those entering fifth grade. Younger kids typically aren't developmentally ready to work as independently and abstractly as we need to them to. If you have a lot of adult or high school help you might be able to extend down to fourth grade. The important thing is that students are successful. If they get it in their head they "aren't good at programming or building" that perception could last for years when it was really just that they weren't developmentally ready.
5) It needs to be fun! If it becomes work or another class then we've done something wrong.
Let me know of you have questions.