I understand your predicament.
Your MIL is kind and loves her grandchildren.
If you can't get your message across to her, then the only way is to advise your boy NOT to take too much fruit juice after a heavy meal with soup.
MIL is a "insurance agent" style of person, you know what I mean?
It takes a lot of strength to go against her.
Ah boy has that kind of strength since young, so I need not worry for him when he is at her place. I guess he takes after her a lot.
I gain this strength after years of obedience and frustration at her hands.
Elder boy yet to master it. He is too respectful to her.