Arduino 3-in-1 kit and Raspberry pi Picar-X - Both Excellent!!
I tried Arduino 3-in-1 kit and Picar-X. Both of them are great and come with great online tutorials. My kid (6yo) did most of the assembling and I went over some code and logic with him. He really liked the Arduino car tracking a black tape/line and played with creating a course with crossings. By editing the logic, we can see how the car follows a different path at the fork. Customizing and interacting with LLM is great. There are many projects in these kits, and being able to modify code easily gives a great learning experience.