Five months is too young, I think. My brother and SIL used to leave their babies with her parents for that long, at that age and even younger, and I could never understand it. There is no way I could be away from such a little one for so long. But then, I am regarded as the overprotective, militant breastfeeder, co-sleeping nutcase in my family, too. So my opinion is most likely skewed by all my wierdness.
That said, my oldest (9) is going to her third straight year of weeklong summer camp in a little more than a week from now. During that week, my in-laws take the other kids who are old enough, which we seem to have agreed is about 3 years. (They are weaned and at least partly potty trained by then.) This year will be the first time for the one who is just about to turn 3, and DH and I will be ALL ALONE for a whole week! :eek: If the little guy can handle it, that is. But he recently stayed for 3 days over spring break with his older siblings up there and did just fine.
The first year, or even two or three, are so short and so formative. That’s the time when attachment is happening, and I think we need to do all that we can to make sure it is very secure. After that, you have all the time in the world between 3 and 18 for them to spend lots of time with grandparents. After their time with my in laws in June, then my kids go spend a week of July with my dad and stepmom, and their really cool boat! So it’s not that I am against these kind of visits, just wait until they are old enough. Then there is no angst, everyone feels secure, and everyone can enjoy the visit and the change of pace without fear, tears, or anxiety.