something appropriate only for children.
something very easy or simple.
Origin of kid stuff
An Americanism dating back to 1925–30
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
Something very easy or uncomplicated, as in That new computer program is kid stuff. This usage alludes to something suitable for young children, or “kids.” [c. 1920]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.