No is the best word EVER. Used correctly, you can do whatever you want, whenever you want. The trick is in your tone. You must not waver. Show them who's boss. It's YOU. And most importantly: never. give. in.
Grown Up: C'mon sweetie, it's time to clean up.
Me: No. (do not make eye contact. keep doing whatever it is that you are doing)
Grown Up: Ha, Ha, Little Man. Let's pick up your toys.
Me: No. (keep your voice level. no eye contact)
Grown Up: Okay, enough playing around...you can watch Scooby-Doo later. We need to pick up this mess.
Grown Up: *sigh*
You've won. The *sigh* is your first sign of victory. The Grown Ups will give up if you are solid in your "NO.", unless they are hard of hearing or just stupid. They should not pursue this any further because they are deathly afraid of your next tactic. If they are not yet afraid, they will be.
What is that next tactic? Stick around. I'll tell you all about it.