Wow, that must really suck. Personally, I wouldn't stand for that.
I'm pre-T right now, so I don't force my family to use the right pronouns, although I did force them with the name thing. But even my LGBT friends who didn't know me "before" slip up on pronouns, and I get it, because I sound so female that their brains wants to say "she." I'm in that awkward in between stage with the pronouns right now. But, as soon as I start T, I'm going to start INSISTING that my family use the right pronouns.
One good tactic I've found is to correct people every single time. So like, if they say, "Can you pass this to her?" You say, "Can you pass this to
him"
People get tired and annoyed with being corrected all the time, but there's only way one you will stop correcting them, if they stop screwing up. And I think they will start to pick up the habit just by hearing you say it all the time.
And when they didn't, I started getting PISSED at them. I would scream at them, "THAT'S NOT MY NAME!" It was one of those moments where everyone stops and stares, because you just screamed. And it probably did me look like a child, but they usually responded like, "whoa, calm down. I'll say it right."
Of course, I would have to say, causing such a commotion in front of your kids would not be such a good idea. but you could still correct them every single time until they get tired of being corrected.
I will have to say, that I disagree with Jasper just a little bit. While I think it is great for your kids to have a relationship with their grandparents, I don't think it's good for the kids if you don't get along with the grandparents. Kids sense these things. Kids know when there's tension around. The kids will notice if you're always in the back room while everyone is visiting. And the kids will end up hearing the wrong name and the wrong pronouns from these grandparents as they continue to grow up. And I wouldn't want the kids to catch onto that habit instead of the other way around.
Maybe you could consider making a deal with the grandparents by telling them that you (and your kids) won't come unless they try to get the pronouns and name right. Not in a mean way, but just in a matter of fact way, because this is important to you, and they need to respect that, or else they don't respect you. And I would even go further to tell them that everytime they slip up while you are there will be one more day you will leave early. So if you planned to stay for the week, and they mess up three times, then you leave after 4 days. I love incentive. It works

Anyways, hope this helped give you some ideas. I realize not everyone is as mean as me. But, if I don't stand up for myself, who else will.
Also, it's not that traumatizing to grow up not knowing your grandparents. My parents had a lot of issues with both sets of my grandparents and they didn't let us see our grandparents much. At the time I didn't know anything about the reason. But really, I was fine. I thought everyone else very rarely saw their grandparents as well.