I normally think the majority of MIL and childcare threads are people over reacting, but I would have been really annoyed at this OP. More so the 'we don't have to tell mommy and daddy everything'. If someone was looking after my child, family or not, I wouldn't want to feel like they were hiding things from me. I've got an uncle who does this with his grandkids, calls them 'special grandad secrets' and it really makes me cringe. Last time I saw them he fed them so much cake they threw up on their plates and then said 'it's special grandad secrets though, we don't tell mom and dad'.
Did you tell them you hadn't started weaning yet? If so then it's very out of order. Either way, I would definitely be having a conversation to explain no other foods until you've introduced them, in case of allergic reactions.
Also, grandparents aren't necessairly doing childcare for free out of the goodness of their hearts. A lot want time alone with their grandchildren and it's a win win for them and the parents (you only have to look on here or gransnet to see that). But even if you're looking after your grandchild for free, it's still someone else's child, you still should follow the parents lead.
You've probably already looked into this OP, but have you checked how many tax credits you would get if paying for childcare? Looked into things like childcare. And I don't think you need to chill out, being a first time mom to one child was stressful enough, I can't imagine how hard it is with twins!