Loki was too busy facepalming to really bother with saying no, especially since Jotnar knew full well how to get treats out of plastic baggies.
"Maybe next time, you can keep that in a stronger box, so he doesn't try to eat the plastic when you're too slow," Loki suggested gently, after he'd reached over to keep Jotnar on a shorter metaphorical leash so that Thor actually had a chance to get the treats out. "And also, next time, try to save telling him you have them until he's actually done something to deserve them, would you?"
Thor got a pat on the head, though, much as Jotnar did, since he was kneeling and therefore his ridiculous height no longer prevented Loki from doing just that.