I would try to find a middle ground between telling my MIL to back off and respect your decisions and letting her rule the roost.
They live far away, and unless they are intending to move back to the same country as you are in, I would just accept their gifts gracefully, use them while they are there, then pass them on to others.
However as it upsets your child to be put to bed by his gran, insist that you do so, but invite her to watch. Likewise insist on him having his usual naps, as getting overtired isn't good for any child.
Suggest politely, that they might want to sit in the garden, go for a walk or take a nap themselves while the wee ones are sleeping in the afternoon.
If they don't take the hint, try to get your husband to talk to them, they are his parents after all. Knowing men, he will run a mile rather than tackle his mother!
Once they have gone home, relax then in a month or so, tell your husband that you find it difficult knowing what to do for the best when his parents are visiting and ask his opinion.
I doubt you are risking divorce by making your feelings known, as long as you do so calmly and politely.
What happens when your parents visit? I hope you have family that comes too.