Usually used in casual conversations, not for formal writing or speeches. I finished the game in one hour, it was kids' stuff.
Pro Tip 2/3
Subtle Mockery
Can imply that the task is not impressive, trivializing someone's effort. He was proud of his win, but the others felt it was just kids' stuff.
Pro Tip 3/3
Cultural Association
In some cultures, this phrase might be deemed disrespectful if used improperly. She called the local traditions kids' stuff, which offended the community.