Surely there should be an element of grandma's house, grandma's rules?
When I was small granny's house was the place we got to eat biscuits in bed, have warm milk and chocolate for breakfast. I'm sure my Mother wasn't delighted but in the great scheme of things it didn't matter.
Was it truly no alternative offered or no alternative taken? I had my friends kids yesterday and her daughter didn't eat much of that through she had chosen it. I also unxpectedlyhadto provide dinner (so it came from my freezer and was a bit random) so not much of that got eaten either. I tried to provide other things without pushing it that I know she likes but they also went untouched, I would be v. Cross if my friend said I didn't feed her.
If a drink was provided in a cup then I don't really see the issue - babying bottles at home. Grown up cup at granny's?
I think you aren't really cross about the non feeding thing but about the failure to respect your rules.
Also don't forget that your MIL is in all likelihood out of the habit of thinking "small child" so may not be tuned into snacks/kiddy popular food etc. I'm not and I certainly don't have cupboards full of appropriate snacks if the ones I do have are being rejected...
My friend is wise though, her kids know there are different rules in different houses so they will not expect to get the stuff they get here at home!