Lake Dianchi - walk around the lake, blow bubbles, maybe take a boat ride (but do NOT touch the water). Nearby is the Hongta Sports Center (?) - but you'll need a car, bus schedules, or know how to call taxis or better still - the uber-china drivers (better service, less spitting, hacking, and smoking, not to mention courtesy).
Green Lake inside - eat at The Park in the center of the Green Lake Park - some amusement rides and games (I think) like bumper cars (but maybe NOT for unaccompanied minors).
The prolific movie theaters with kiddie focused movies.
OUTSIDE Kunming - there's a difficult to find international youth hostel - and nearby is a large park with lots of kiddie things to do - more blowing bubbles.
Wild animal park on the NE edge of kunming.
Kunming Zoo. Despite the detractors - it's inexpensive enough for many rural people to visit, has a rather expansive amusement park in the center of the park (on top of the hill) and there's a MacDonalds inside. Yes the people are grubby - but they're not well trained in the art of litter management, spitting, hacking, smoking, and personal space invasion. Adults from western developed countries will be appalled - but the kids probably won't notice if this is their first visit to the zoo...and any other zoo in any other developed country will be a major improvement for your next zoo visit.
FuXian Lake - beautiful clean water lake - kids can wade - but be vigilant! Sea food, Hilton Hotel nearby if money's not an issue, for their lunch buffet.
You can also pick up some inexpensive kites and go fly at the larger parks - just be prepared for kite-eating trees and overhead power lines - trees not so much as the cold snap radically pruned many of them.
Finally - for the ultra low-budget - any of the major shopping malls - there are abundant toy stores - some even have samples that kids can demo - just make sure they don't try to ingest product.