I read this as Granny deliberately asking the kids in front of mum so that they do get excited and mum can't then say no. I think it's manipulative and underhand and it would piss me off tremendously. I don't think you're being unreasonable at all.
I was at a friends house recently for a play date and her SIL turned up and told her sons that they coudl come to hers for tea that night (big treat for them) my friend says, yes we'll arrange that sometime but not tonight, SIL says, no, I've already told my son that we can do it tonight, I'll take them now...
You could argue that was nice of SIL to offer to take the boys for tea, or you could see it as her running roughshod over friend by getting the kids to agree to something that she wanted to happen and not listening to the subtle hints my friend was giving about not wanting them to go.
As it was she took them, made no comment or observation about me being there and left with the boys and my friend felt exceptionally awkward and uncomfortable - but what could she do? The offer had been made directly to her son by SIL in front of her? Yeah, she could have said no (and I think if it happens again she would) but the repurcussions would have been distraught son all afternoon.
I'm all for addressing questions to kids directly, but I think there is an aspect to this that smacks of granny trying to blackmail mum into saying yes to something she wants to do. As fluffy says maybe you have to have been in that kind of situation to recognise it.