As background: It is Lent and we are very observant.
My Dad is here, as he is every day teaching the boys. I was setting the table for lunch and I put out a jug of water. DS2 (just 4) starts whinging that he doesn't want water, he wants juice. Now, to me he is just being a pain because we don't have juice with meals as a matter of course anyway, he has been very willful over the last few days anyway, because this is the first Lent that we have made him do 'properly' in that we have said no to anything sweet, fried or leaven at all and no red meat. I don't think this is a problem, his brother is just 6 and has done it from the same age. Anyway, so I serve the meal, veg broth and Matza. DS2 is still whining and refusing to eat, saying, 'I'm too thirsty, I'm too thirsty, Meenor!' His name for my Dad.
Dad then says to me 'How can you see your child suffer like this? I never denied you a drink as a child.'
I answered that I am not denying him anything, he is choosing not to drink the water and is only playing up to an audience.
A little later, [DS2still whining] I nip out to the loo, and come back to find DS2 has got a glass of squash! 
AIBU to think that my Dad should have stuck to my rules in my own house?
FYI, kitchen has been cleaned for Lent, so he had to purposefully walk through to my office the garage to get the juice from the child inaccessible cupboard!