This thread confuses the hell out of me!
It was well documented in the press at the time that the children had been 'camping' in the garden to give them something different to do after Michael 'died'.
They camped with their cousins and I guess it was supposed to be a diversion for them and a bonding experience with their cousins, who although they knew, they had not lived with before.
From a hoax point of view it was perfectly fine for Katherine to pop out to the shops for camping equipment, but if you are of the view MJ actually died, yes it would be a bit odd she had to go herself, but you could also say she was throwing herself into being a good grandmother and guardian for the kids and needed to do something rather than sit in the house weeping.
I imagine she's a very strong lady and so even if I believed he had died, I can totally see her being able to do this for the children.