It starts off the hop that they should be scared of the thing they are trying to do. Kids don't hear the implied stuff you might mean when you say that to an adult that is cutting wood with saw, for example.
You want your child to figure out how to do it, not to be scared of doing it.
"God be with you" seems like a positive statement, maybe I'm wrong there. I also can't recall the last time I heard that said.
Helping the child wrap their head around doing risky or dangerous things in ways to reduce risk is very handy for them as they get older and have to make choices and navigate stuff when they are starting to become more independent.