Thanks all, especially atomicbadgerrace, assuming you’re telling the truth and are not the proverbial dog on the Internet.
Of course, in this case, if you’re all wrong, it’ll be my friend who gets the shaft, not me. 
Residents of Guam, as I learned from Stephen Colbert’s interview with the governor of that island, are known as Guamanians. The indigenous peoples of Guam are Chamorros.
(“I’m sorry, you have Guamania. It can only be cured by eating Chamorros.”)
As someone who has done it a lot (and even not being a US citizen for a while), you don’t need a passport, you won’t go through customs.
Even travelling into the US from another country with a stop in PR, you do customs entering PR and then no more in the US. San Juan is your port of entry.
I am not sure about the kid. Both mine have had passports since they were weeks old and we carry them as their ID since they don’t drive.
Movement of people between Puerto Rico and the mainland USA is the same as movement between any two states. No passports, no customs clearing, agricultural/health inspections may be called for. (And yes, KG, we could and maybe SHOULD have long ago done like Cali and Hawaii and required APHIS inspections BOTH ways… too late for that, may have saved us some exotic-species infestations…) When minors are involved there may be some specific requirements established by the airlines as to documentation.
Actually, the only polities that* legally *are sovereign “states in association with” the USA are the former trusteeships of Palau, the Marshall Islands, and the Federated States of Micronesia.
The Northern Marianas, Puerto Rico, Guam and the USVI are all legally unincorporated organized territories: “belonging to, not constituent of” the United States – a *jurisprudential *classification, handed down by SCOTUS a century ago, not set forth by statute, and which tends to be confusing as it means the Congress can legislate differentially for each case (PR is exempted from much of the US tax laws, USVI is a Free Port outside the US customs zone, Guam and NMI have different labor-visa requirements, etc). Within that classification, PR and NMI are organized as “commonwealths” meaning that their *internal *organization is accomplished through the adoption of a homegrown Constitution ratified by Congress, as opposed to organized through a direct Act of Congress. Natives of all four are US citizens. (American Samoa is a unincorporated “nonorganized” territory, which does NOT mean it’s not organized, but that the Congress delegates much of the organization of the territorial government to the Dept. of Interior; AmSamoans are NOT born US citizens.) In the intervening century, Congress has legislated to make the various territories “US soil” for a whole set of additional criteria – for instance PR is US soil on equal footing with the states for purposes of nationality and citizenship since 1941 – so the “not a part of” bit by now is pretty damn thin and barely means they retain the right to choose independence.
In its official Spanish styling, the Puerto Rican polity is referred to as “Estado Libre Asociado” but that’s really more of an ***aspirational ***title: the founders of the commonwealth and those who currently lead the party that founded it intend that it some day achieve the condition of being a sovereign state in association with the US. It’s nowhere close yet.
Oh, and sorry for the doublepost but it crossed my mind too late: Normally if I travel to the mainland I will just use my DL and only take my passport if I’m going to be close to the Mexican or Canadian border, in case the chance of a daytrip arises; I’ll probably get myself one of those new passcards for that purpose. But in the last year I’ve taken to using the passport also if I’ll be traveling to some location that does not see much PRican travelers, since my DL is getting close to expiration, looking quite ratty, and is not RealID compliant (the new one this year will be).