kids' stuff
1. Something that is very easy to understand, achieve, or use, especially when encountered by one on a more advanced level. The highly anticipated match turned out to be kids' stuff for the returning champions. Do you have anything more difficult? This book is kids' stuff.
2. slang Marijuana (due to it being seen as a "soft" drug). If he's just getting high on kids' stuff, I wouldn't be too worried. At least it's not hard drugs!
kid stuff
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]
kid stuff
n. marijuana, a drug for beginners. (Drugs.) He’s still using kid stuff.