1) At the most basic level, probably, thousands of 5-7 you can meet, or even less.
2) There are plenty of ready instructions.
You can make stupid guides, but a basic knowledge of electronics and programming skills are welcome.
Where to get information
books, youtube, google
and order everything you need?
ampere cheaper on aliexpress
5) easier (but not cheaper) — lego mindstorms
if the child is of appropriate age, it will be interesting and useful for myself along with my dad to assemble and program the robot.