This idea was rejected way back in 2014 based on details in this thread: https://community.atlassian.com/t5/Confluence-questions/How-to-break-the-children-display-macro-into-two-columns/qaq-p/657107
I think it has been long enough that we can ask for this feature again.
Child Page List macro - add parameter for number of columns (default 1, max 4)
