Do you have anything like the TV programme How Clean Is Your House? in Australia? I'd get Kim and Aggie to clean the house before you went round LOL. (Maybe you can google it or check out their website.)

I know you don't want to deny them seeing their grandchild, but your little lad is your child more than he is their grandchild IYSWIM and you really shouldn't have to put him in a potentially unsafe situation just to keep on the good side of the in-laws.