I agree with Lach.
I think you have to consider how you want to have people/children over in your home. If your plan was to play outdoors and you were inviting them along, that needed to be clear in the invitation. "We'd love to have you come play outside." Part of having people over involves having your plans disrupted. If you didn't want to watch a movie, that is fine, but it needs to be made clear with a simple "No, we're not watching anything today" -- not by making the viewing unpleasant.
If you flip it around, imagine if a parent invited your kids over thinking they would watch TV all afternoon and your kids were asking to play with toys that were on shelves, and then that mom huffed about it and made it difficult to play with all the pieces (to avoid mess, say). It's the same thing. It's just that you have attached a value to television.
I think you taught your kids a lesson in inflexibility and rudeness. I totally get that it is a charged situation for you, so I agree that you need to find room for your MIL in another way.