I recognize that your situation is a little different, as you live in walking distance, but if Hubby and I ever have a baby, I'm going to ask ILs to wait until baby is at least 3 months old before coming to visit.
Before I get flamed, let me explain. We live in Canada. ILs live in the UK. They are lovely people (with the exception of FIL with whom we have a minimal relationship for various reasons; he will not be meeting any future children) and I would never withhold my children from MIL and SFIL. Even so, the germs in the UK are different than the germs here, and let's be honest, airplanes are a glorified cesspool. I have frequently became ill in the past after flying. I'm not suggesting that MIL and SFIL are The Great Unwashed or that British people aren't clean. I've been to Britain in the past and loved it. But I don't want my LO being exposed to germs from another continent until after first shots have been given.
People have different feelings on when they want others to meet a new baby. Not to mention that, really, the time they spend with others and who else has already seen the baby are, quite frankly, not your concern. Saying "well, so-and-so has seen the baby already and I haven't" sounds similar to an adolescent saying, "but so-and-so is allowed to have/wear/eat/do whatever, why can't I?" If your children behaved that way or would have behaved that way when they were young, what would be your response?