J.
Hi A.,
Typically, if cats are friendly and like children, they will come up to them and hang around them and want to play. If they are not used to children or don't like guests, the will usually hide or avoid the new people. If the cats want to be left alone or are hissing, make sure your son leaves them alone. It seems to me that most children are fascinated by animals and want to touch them and chase them. Keep a close eye on your son and don't allow him to chase after or corner the cats. If a cat feels cornered, they may strike or bite to get away.
Ask your mother-in-law to trim the cats' nails before you arrive and if they go outside at all, they should be dewormed. Also watch that your son doesn't play in the litter boxes (a favorite game of my daughter). Make sure the cats are not in the room where your son will sleep and close the door before he goes to bed. I doubt the cats would do anything to him, but it will ease your worries.
It will probably be a good experience for your son. I recently opened a cat veterinary clinic and my 3 year old daughter loves helping feed and take care of the cats. She knows instinctively which cats to avoid it seems. Cats usually are good at making their feelings known.
Good luck and enjoy your vacation,
J. Karnia, DVM
Scottsdale Cat Clinic