I told my kid he might be kidnapped, but would be returned eventually.
How did you know he would be kidnapped?
I have a bet with a friend over who could terrorise a child the most.
It is fun to play around with kids but some pranks can be harmful. What you insinuated could have really traumatized a child, and you should have been more kind.
I was only pranking.
I'm glad to hear that this was just a joke, and that you weren't actually planning on staging a fake kidnapping, however this is still over the line for a prank.  You should not be trying to scare children, even if you think it's funny, because it is traumatizing to them.