You might want to look up botball. It will probably be more appropriate for your fifth graders. It gives them a good challenge, and most students were somewhat successful. We do robots that follow lines using a light sensor and sumo robots, but this is in the high school level, rather than K-5. You might try making the basic frame for the robot out of legos and letting them modify it rather than designing the entire robot.

It gets better than that. Much of the crap I have had to implement will show the expected improvement that my students ought to receive by some bar graph of overall improvement demonstrated by the program. In fine print it says that the measured improvement was for ONE student. Most teachers and administrators fail to understand why one student's success with the program does not mean all students, or even some, students will receive the same improvement.