Yes, those things all seem mildly annoying, and I know you're claiming you don't let your kids onto your frustration, but I feel like this is what is still really rubbing me the wrong way:
So this year MIL asks if we're planning a party for DS who will be 10, and I simply stated that we didn't do big parties anymore, and just let the birthday child pick out what he or she wants to do for that entire weekend instead.
Do you really think a 14 year old isn't picking up on this? ..... your reaction to it doesn't seem to be "punishing" your MIL (not your job, anyway), but your kids. Plan what you'd like to plan, and just assume she may be flakey. Don't let that ruin your time, and don't let it ruin your kids' time.
I think the OP's reaction is absolutely perfect. It sounds like the pressure to have a big gathering came from the MIL in the first place. So now they just don't ever give in to the pressure, and they indulge the b'day kid some other way. Some way that the b'day kid prefers.
Of course the kids have figured out that Grandma is flaky, and blows off their time/energy/events. They're probably relieved to not keep having parties that more than half the guest list disrespects.
As I pointed out, I responded as the child of a mother who did this kind of thing in response to a grandmother who acted similar to the one in the OP. I just let the OP know how *I* felt, as the child in this scenario. People are free to disagree with me, if they'd like, and maybe I would even do as the OP is doing, but I'm finding it kind of annoying to keep repeating the fact that my responses are based on someone with actual experience as the child in this scenario, rather than just speculation. People seem to overwhelming disagree with my experience as to what a child *might* feel, but that doesn't discredit that that is the way *I* felt, and is likely the way many children feel in this scenario. Sure, it might not be entirely fair, but it's still a valid opinion, and something that definitely should be considered. No matter how much other people want to cheer on OP's actions, does she really want her child to end up being the one who feels bad in this scenario? Even if it's not entirely fair or logical (children rarely are), it's still something to keep in mind. Obviously, OP has her child's best interests at heart, and I definitely thought it was worth mentioning how it can often feel as the child in these situations.
I appreciate your response.
I was also a child in a similar situation-- my Grandmother always gave preference and priority to my cousins over my brother and me (which stemmed from priority to my uncle aka the Golden Child, over my mom aka the Daughter She Never Wanted). Her actions became more and more obvious to me as I grew, without any hints from my mom. When I was around 12 my mom finally addressed this behavior with me directly and it was a great relief. It validated my feelings and made me see that my brother and I weren't alone in this second-class treatment (because I hadn't really noticed that my mom was on the recieving end of it too). My mental approach to family gatherings changed: plans were now made with the understanding that Grandma may or may not be there, she may try to prevent our cousins from joining us, and we could not change her. It was empowering in a way, and I no longer had some subconscious sense that my mom was failing to make everything work out.
Over the years, this has been a source of bonding for me and my mom, and over more recent years we have been able to recognize the problems we have had with other relatives really weren't their fault, but Grandma's fault.
So my experience was different from yours. They are both valid. I guess that's what makes it so hard to know what's right, when you don't want to damage the relationships
your kids have with other relatives.
Despite our different experiences I do agree with you that the OP should plan what she wants whether it is a trip to the amusement park with 3 buddies from school or whether it still is a big party. Do this with the knowledge that MIL will probably flake out. I hope the OP's kids will be able to share this mindset.
If the OP chooses no more big parties (and it really is what the kid prefers) I think the OP can deliver her message to MIL in a way that conveys "yep, this is just how our family events are evolving" rather than in a way that says "you're so unreliable we're doing this to spite you".