How can I retrieve the execution status of parallel triggered child jobs to a pipeline script
Use the result property of object returned from the build step.

This was already asked and has an answer over on StackOverflow.  Additionally, this is hinted at in the official build step documentation in the description for the propagate parameter ("use the result property of the return value as needed").  I haven't tested it as I don't have a test Jenkins instance available right now, but based on those two pieces of information it looks like something like this should do the trick:

def b = build('my-job-name', wait: true)

def r = b.result