When I was a child in the 80's in Brazil this was still widespread. I remember walking over a relatively short friend of mine (we were both around 8 years old) and his grandmother physically made me step back over him to undo it, so it wouldn't prevent him from growing.

It is not just Kazakh, but also South Asia.

It is superstitions like this that leads me to think somebody must have made up a reason to little kids how they should not cross over a child as a matter of manners, but wrapped it in a story that led into adulthood.

If you are as uncoordinated as I am, that may be correct ( I'm assuming that an adult falling on a child could do enough damage to stunt growth ).