You can never be right for your MIL and the way it was done by her, when she was having her kids is the ONLY and the definitive way of doing anything you might mention. Tell her if you cant ring and check up on your kids then you will be leaving them with someone else.
I had to stop my MIL having unsupervised time with my kids because she was feeding them bad foods (beer, boiled sweets, really salty foods, chocolate for lunch) and letting her play near a 4 ft step unsupervised, and near the pond, and smoking in the same room, and letting people I didnt know look after them while she went out or did something else. She was putting them in their high chairs for hours and leaving them there while she cleaned house etc, or forgetting to feed them lunch till 4pm.
Hold your ground! Do it YOUR way they are YOUR kids.
If she is the only person you can get to look after them, then change your life around until you can sort it out another way, just so SHE cant have her own way. Let her have her own way once, and she will just take over. She will want it ALL her own way.
My MIL told me a little story about how some woman had her kids taken away because she didnt mop the floor every day. Right after slagging me off for not mopping my floor every day and me telling her to butt out. Basically she was threatening me that if I didnt do my housework the way SHE wanted it done, she would report me to social services and try and take my kids for herself. Maybe yours isnt that bad, but even so dont let ANYONE walk all over you where it comes to your kids. If you cant even call up to talk to them or check on them you dont know where she has taken them, or what she is doing with them. WHY does she need so much clear time with them where you cant get to them or find out if they are ok? What is she doing to them that she dont want you to find out about???????