Writing 'The Rules' for grandmama sounds so - well - official. Surely OP's son could have softened it a bit, by just telling her, for example, 'oh, by the way, little one does enjoy playing with knives and matches ... and best not give him gin too often, as it upsets him tummy and when you send him out to play on the motorway, make sure he's back home by 11 p.m. ...'
Seriously, I've looked after all five of my grandkids, from babyhood onwards. My daughter, and daughter in law, have mentioned stuff they'd prefer I do when caring for their children and I've never taken offence. But I'd probably have rolled my eyes if I'd ever been presented with a large-type document titled The Rules, though of course I'd have abided by them. I am, to be honest, trusted to know how to care for the little ones. To be sure, there've been a few hiccups - such as when I was demonstrating how to use sealing wax to the four year old. The wax spluttered and a molten glob flew onto back of child's hand. Small burn, plenty of cold water, a few tears but I was totally mortified. Daughter, bless her, said that accidents happen and how nice it was for little one to learn such useful things as how to use sealing wax (no, she wasn't being snide 
Little four year old is STILL announcing to anyone she meets that 'grandmamma didn't REALLY mean to burn me ...' 
I think OP's son should chill a bit.