@CockysGirl YANBU.
I mean it may seem a bit petty to some, but it's not. And the grandmother's reaction was pretty awful, and set a bad example to her grandaughter. 'Nasty lady won't let you walk on her wall!' What a horrible, and very childish thing to say.
She has no right to say you are 'nasty' for not allowing her grandchild to walk along on your wall. And as you said, 4 foot high is quite high, and if the child fell, you can bet you would be held accountable somehow (by the entitled granny!)
Can't believe the amount of people thinking it's fine for some random child to walk along someone's 'privately owned' 4 ft high wall, and if the owner of the wall complains, they are 'miserable fuckers,' and should 'let kids be kids!'
Yeah it's fun to walk along a wall when you're a child, but most people would have the respect, and common sense, to not let their child or grandchild walk on someone's privately-owned garden wall!
If the wall is OWNED by someone, then be prepared for your child (and you!) to be told off by the owner. They are within their rights to ask you to make your child get off the wall!
And don't be a dick and call someone NASTY, just because they ask for your child to get off THEIR WALL! This 'granny' should have apologised to the OP. Awful woman! 