I have to agree with other posters who suggested 2 trips. When your mother and MIL watch your son, they each have him to themselves. Although they know that another woman hugs him and plays with him on days he is not with them, they never have to "see" your son with each other. I would be afraid you would have to run interference the whole trip. To make it fair, you could plan your trips well in advance, so even though 1 grandmother goes first, the trip for the other is already planned, so she won't feel slighted. The added bonus for you is obvious: 2 trips for your family(consider purchasing AP's for you and DH), 2 nights with DH alone(let Grammy spend 1 night with DS on each trip), and you can concentrate on thanking each grandmother individually for their contribution to your family.