jgnat that was good advice.
The MIL should be reminded that you are the head of your family. That even she (as a woman) is subject to you. That by blatantly putting your desires for your children aside she is disrespecting you. There's no way you'll convince her that she's wrong. Remember JW's are taught that they are special because they have the truth. Obviously, you are not special enough.
You also might mention to her that if your children are sodomized or raped by some JW while at a bbq or meeting she will be responsible (blood guilty) for the rest of her life, does she want that? Perhaps even the courts would find her guilty since she has specifically been told NOT to take your children to those get- togethers. How about waiting until they are at the age of 11 or 12 then take them to a meeting. That way they'll be old enough to tell you they don't want to go to that boring KH.
I never talked to my elder/ husband about the falsehood of WTS. He eventually asked me questions and read Crisis. He gradually stopped attending meetings, quit eldering and became a whole man, the one I originally married. So I think everyone needs to find out for themselves how horribly they've been duped. Only then does the new light falicy really explode. Kaaboom