Mike, I don't care how much Patterson blood is in you. That is NOT the way you go about it. First of all, you haven't even discussed anything with Mr. and Mrs. Grouchypants (tm somebody else). And if you had, it should have been in the nature of "Get a grip." And "We have a six-month-old baby; do you think your THUMP THUMP THUMP is going to help him sleep?" At any rate, you do not put the responsibility on Merrie's tiny shoulders.
There is a limit to how quiet is "possible" with a 2 1/2 year old. I can understand "No jumping on the bed", because it's brand new. I can understand "No running around", because that drives parents up a wall, let alone neighbors. But the process of conditioning a kid means the slow, painstaking process of differentiating between acceptable and unacceptable behavior, and squashing the unacceptable while allowing the acceptable...because she is a KID! You do not, at her age, hit her with, "The neighbors are more important than you are and if you make a peep, they will come upstairs and eat you."
When I was little, I read a book of poems entitled "The Man Who Sang the Sillies", by John Ciardi, whoever he was, except he wrote this. Anyway, I remember one couplet: "Whenever Silence sits down with boys/He looks over his shoulder and here comes NOISE!" And these days, since society no longer forces girls into the Little Lady box, it applies to both genders. Even Amy Vanderbilt said in her etiquette book, regarding being a guest in a house with children, "Normal, healthy children must make a certain amount of NOISE. If you can't handle it or the parents can't isolate it, go to a hotel." (I just feel I need quotes to back me up.)
That said, that is one spacious kids' room, as seen in panel 1!
ETA: Oh, and now we know for sure: the Grouchypantses are in Mike and Dee's old apartment. So I wonder, did the Professors make any noise in the creation of the unholy mess they left, and it didn't occur to M&D to be bothered? Or did the Professors spend most of their time in a stupor on the couch, IYKWIM, and only get up to order pizza and answer the door?