I hate to say this but I do think you are being completely over the top and controlling with regard to your ILs and what they do with your children, and wonder if you have other issues that you're projecting onto their relationship with you/the children?
Or a particular reason to worry about your childrens' wellbeing, because what you've posted about them seems utterly reasonable and normal.
I am overjoyed if my ILs take DD for a walk when they look after her, she loves it and so do they.
DD was going headfirst down a slide from about 18 months (OK, she's an adrenaline junkie but still!) and adores being flipped over by her arms - at 4.3 she still has them safely in her sockets!
Our door is never locked if people are in the house. DD can open it (just) but she knows she's not allowed to and doesn't.
Giving them something different for lunch (unless there are allergies involved - and I do think the chocolate thing is bad) is not the end of the world, unless they're feeding them the caviar and lobster you were saving for yourself...
Of course, if you feel they're deliberately going against you (wrt the food issue) to wind you up, then that needs to be tackled and preferably by your DH (he ought to be very concerned that his child is being given stuff he's allergic to!).
I understand why, after a big row, you're wary of them, but to not trust them thereafter with your children as you feel they'd not like them because 'they're a part of me' is presuming an awful lot and seems to me a little paranoid? I'm 99% certain that both sets of GPs in our family see DD as 'their' grandchild and very little to do with me or DH!
It just seems such an overreaction on your part to think that the easiest way out of this situation is to divorce your husband. In fact I'm wondering if (and forgive me if I'm talking out of my arse, I usually am) you're not depressed, as these sort of things (divorce, walking out) seem like fantastic solutions when you're so far into your own problems that it all seems insurmountable.
Perhaps if you talked to someone else (friends, family, perhaps your GP) about these feelings, you could get some distance and look at things in perspective, because from your posts you come across as very het up over this situation and perhaps not able to see the wood for the trees?
Take care of yourself, because you sound pretty low. x