Is she eating while this is going on, or just playing? At that age, Tyler had a tendency to start messing around when he really wasn't hungry anymore. That was my cue to end mealtime, even if he hadn't really eaten very much. While I think that some goofing around during meals is okay, since toddlers are still learning so much, there's a line between experimenting (squishing, seeing how gravity works) and boredom. Where you draw the line is up to you.
Also, something else that made a big difference was keeping an eye on when Tyler drank milk. He drinks as much as your DD (and would drink more if we let him), but we have to be careful not to offer him milk too close to mealtime or else he'd fill up on that and not be hungry. HTH!