once your little fella is old enough to visit with them alone, perhaps you could make a list of the food he is to eat (like give them options for brekkie, lunch, dinner & snacks) - or even supply the snacks etc and tell them he can only have water to drink... and supply your own sippy cups/plates/spoons tec if you think theirs might not be clean

as for when you all go to visit, maybe bring a blanket from home (or two) to put on the carpet for him to play on - im not sure about once he is crawling, as he will be off to explore!

maybe dp could ask them to vacuum before you come as your worried about alergies from the animal hair.

not sure if that really helps, but it might just be a few ideas!