Hi, E.,
Wow - it's so hard, isn't it? I remember every time I'd leave my first child with my own mom (who loves him every bit as much as I do) that I would drive up the street, park the car and cry for five minutes. It wasn't that I worried about her ability to care for him.....I just missed being with him and not knowing every little thing he was doing. That's my baby boy, after all.
My husband and I are pretty strict about what our children eat, what they do, how much TV, etc. But I have always wanted my children to have a special relationship with their grandparents, and to have wonderful memories of them after they are gone. So I knew it was important to let them live with Grandma's house rules while they stayed with her. Those are the special things they will remember, because it was different than Mom and Dad.
So, when we are at Grandma's to drop them off, and my son asks for a cookie and I say, 'no, you don't need a cookie this close to dinner' and I hear my daughter whisper to him, 'Cade, just wait til Mom leaves and ask Grandma; she'll let us have one!' -- I pretend to not hear, and just smile, because I know it's true, and in the grand scheme of things, it doesn't matter if they have a cookie before dinner. What does matter is the memories and bonding they are creating.
Really, even if she fed them ice cream all day and let them stay up until 11:00pm, in the long run, it is not going to hurt them. (and you know she is not going to do that anyway) You already said you know she would never do anything to harm them. So trust your own judgment and trust her.
As someone already mentioned, make sure your MIL has your number with her for anyone to find in case of emergency. Better yet, get those little bracelets for your boys that list ICE numbers (yours). Then, if something were to happen while they were out, anyone who found them would know who to call. But statistically, the chances of an emergency happening while MIL has them are not that high. Not as high as our mommy worries let us think. :)
Then, just call a couple times a day (mid-day, before bed) to talk to the boys and that gives her a chance to tell you about the exciting day they had.
I know, truly I do, how hard it is. But they will be okay....and so will you.
Blessings, J.