I am also an adoptive mother of 4 children , 3 with disabilities (now adult). I also foster 3 children on a permenant basis. I have been doing this for almost 30 years now.
I do agree that sometimes my children have had to be parented somewhat differently than "the norm".
Not knowing your children were adopted I read your thread totally aghast at the situation of your own making. I found myself getting quite angry that you have created a situation that you are now blaming others for. You honestly have come across almost deranged and certainly a complete control freak. I think you behaved very badly towards your MIL.Though of course I am only going by words on a screen that I maybe interpreting incorrectly.
Then I read of your children being adopted, and I am even more aghast. I find it so amazing that you would make it difficult for your children to see the grandparents they love, and who love them. So many adoptive parents long for their extended families to love their children too, and to play a part in their lives. How many adoptive parents would long for their new aunts and uncles to buy them Christmas presents? Long for their own parents to be loving grandparents to their children? The fact is no matter what past your children may have, and no matter what their disabilities are, they have (obviously) lost their birth families. Now you are making it difficult for them to be with the new grandparents they love. Having the children welcome the grandparents (or other relatives) to their new home, showing off their things, bedrooms, by having the grandparents come to them is much more important than a day out somewhere. I am sure you must know this already through, it is part of the bonding process, and very important. So doing what you are doing is, in my opinion and from my own experience, quite cruel to your children and depending upon the age of the children potentially causing them much emotional stress. They have lost one family and it is so important for adopted children to have regular access to their extended adopted families, showing them the grandparents are here to stay. I am shocked you are making the grandparent/child meetings to be more awkward, less frequent, and making rules for what seems the most minor of "rule breaking". It is so important for the children to see their new extended families in their new home. And if they became so distressed at having to wait while grandma chatted to the neighbours (a thing my parents and ILs have also done) then it it YOUR JOB to reduce that distress. You are their mother, you know their past history and details of their disabiities, and you are therefore the one that needs to help them to understand, grandma having a chat before she enters is acceptable and quite normal. It isn't grandma's job, it is yours. Part of the package of us adopted parents, especially those with children with disabiities, is to help the children to fit into a world they sometimes find difficult. We can't change the world for them, we can't change other people's actions, but we can help the children through things they find difficult, such as waiting for grandma. After all when attending school they will need to learn to wait a little.
I am also wondering what the social workers who you may still have in your children's lives (if you have a new baby) would think too. Of course we have to protect the children maybe a little more than the norm, but why are you depriving them of new loving grandparents for little reason? I am sorry OP, unless there is far more to this story, I find your actions not only unreasonable, but also very cruel to both children and grandparents.
However, despite my strong feelings I hope you manage to find a away through that is acceptable to everyone, and can once again welcome your children's grandparents into their/your home. Honestly it is really important for adopted children to be able to do that, show off their toys and bedrooms and so on. Good luck!