Juan - did you see the article in this month's Leading & Learning Magazine? The article is titled Lego Robotics: STEM sport of the Mind
This is a good place to start and see what others are doing. I know the elementary school where my children attended offered this as an after school club and they have been quite successful in being competitive and having a mixture of new and returning kids to the group.
Do you currently teach robotics in school? Or are you thinking about getting a group started?
I have looked at Lego robotics but have not started using them yet. Until STEM becomes a part of the core, I do not have administrative support to make such a big change. If I was able, I would quickly add this to our instruction. Lego robotics is easily understood by students and they have a lot of fun with it.
I am not sure if you have a 4H club in your area but they have a fabulous robotics program geared for "after school" 4-H Robotics Curriculum | National 4-H Curriculum | 4-H
You could get some ideas from their curriculum.
My school had the 4-H program several years ago. We had one teacher who used the 4-H Robotics Curriculum using Lego Robotics. Students were VERY enthusiastic for that club. Usually he had to remind students multiple times that their club time had ended (NO ONE WANTED TO LEAVE!) Unfortunately, we lost our 4-H program .