Luckily, I met a renowned robotics educator, Charles Chen, at my son's robotics camp and talked about this problem. He gave me a simple solution for kids age 7-10 before using LEGO Mindstorms:
有幸的是，我碰到了休斯顿地区知名的机器人指导老师 Charles Chen，并和他讨论了此问题。他的解决方法是下面这个简单的公式：适于7-10岁孩子未接触 LEGO Mindstorms:
Practical LEGO Robot Kits = Isogawa's LEGO Idea Books + LEGO Education Sets
Isogawa's LEGO Idea Books
They are different from regular LEGO sets without step-by-step instruction images. In fact, Isogawa's Technic idea books only take 1 or 3 pictures of the assembled ones. Your kid needs some guess and try to duplicate them. Reverse-engineering is the next level of LEGO building for your kids.fundamentals of Machine Elements: gears, shafts, pulleys, turntables, connectors, etc. Great intro book for LEGO Simple Machines Set 9689 and Powered Machines Set 9686.
a collection of hundreds of mechanisms for cars, trucks, motorcycles, and other vehicles. Extension of book 1. My son's favorite.
a collection of hundreds of working examples of simple models. Advanced book, but my son is not very big on this book.
showcases small projects to build with gears, motors, gadgets, and other moving elements. Again, a perfect book matches Sets 9689 and 9686. Mr.Chen uses this classic book to teach his robotics classes.
One step forward - use basic knowledge from Book 1 to build cars. Mr.Chen develops some of his robotics curriculum based on it.
LEGO Education SetsLEGO Education Sets are very popular in STEM society. Numerous schools and camps have been using them for their robotics curriculum. Interestingly, Lego never openly advertises their Education Sets. In fact, you can not find any on their official website. Shrewd marketing strategy - If they do so, the sales of other commercial packages (Chima, Star Wars, etc.) will be damaged.
Friendly reminder - Do NOT buy any LEGO Education Sets from Amazon sellers, much higher price than LEGO Education site.