One strategy that might help the LW, assuming she wants to change her mother's mind rather than draw lines in the sand, would be to enlist her help in selecting treats.
Maybe a script like "I know you think I'm being overly harsh on [Son]. I'm really not trying to deprive him, though, so I thought maybe you could help me figure out treats he *could* have" and then start brainstorming ideas and recipes together. That would help the grandmother see the more complicated side of managing [Son]'s health, and might increase her buy-in to the whole idea.
Writing that, actually, makes me wonder if part of the problem is that it's really all about the kind of grandma that grandma wants to be. If LW's mother was looking forward to being the generous much-loved Cool Grandma, having her daughter say "no you can't give him that" feels like a personal attack, no matter how rational the reason.
If that's the case, maybe LW could follow up with "You're right, it's important for [Son] to have some treats in his life. Maybe there are non-food treats you could bring him, if you want to spoil him a bit?" and then they could pick out some category of treat that only Grandma gets to give him? (A particular range of toys, or tiny lego sets or whatever.) That way grandma gets to feel like Special Generous Grandma and LW doesn't have to worry about what she's feeding Son... and Son gets cool Lego. :)
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Maybe a script like "I know you think I'm being overly harsh on [Son]. I'm really not trying to deprive him, though, so I thought maybe you could help me figure out treats he *could* have" and then start brainstorming ideas and recipes together. That would help the grandmother see the more complicated side of managing [Son]'s health, and might increase her buy-in to the whole idea.
Writing that, actually, makes me wonder if part of the problem is that it's really all about the kind of grandma that grandma wants to be. If LW's mother was looking forward to being the generous much-loved Cool Grandma, having her daughter say "no you can't give him that" feels like a personal attack, no matter how rational the reason.
If that's the case, maybe LW could follow up with "You're right, it's important for [Son] to have some treats in his life. Maybe there are non-food treats you could bring him, if you want to spoil him a bit?" and then they could pick out some category of treat that only Grandma gets to give him? (A particular range of toys, or tiny lego sets or whatever.) That way grandma gets to feel like Special Generous Grandma and LW doesn't have to worry about what she's feeding Son... and Son gets cool Lego. :)