Your granddaughter's house sounds exactly like my parents'!
My maternal grandmother and aunt, who lived together and kept an immaculately tidy home, were wise enough to say nothing.
My paternal grandmother make unkind remarks about my mother's housekeeping, both to her and to me, when I was old enough to understand them, and did not exactly endear herself to either of us by doing so.
A dear great-.aunt of mine frequently offered to do the mending, an offer my mother accepted gratefully, so that got rid of one of the untidy piles of things left around. I took over the mending when I was ten or so and bored stiff during a school summer holiday. No-one thanked me, and after that it was just assumed to be my job.
I have always kept a far tidier home than my parents did, which is probably because my childhood home was such a mess, and the approach of Christmas, Easter, birthdays or visits from relatives drove my mother into what in Scotland is called " a right state" and saw my sister and I conscripted to tidy up.
Either you say nothing at all, or you ask politely if there is anything you can help with. If your granddaughter wants help, she will doubtless ask for it, as long as you neither look nor speak your astonishment of the mess.