Also, how about the idea of moms initiating the trade. or "son swap"?

Yes, for me, age difference in the core of this fantasy. You can view it as a proxy momcest fantasy. Sometimes you consider fantasies of your mom wrong so you project them on another figure your age. Maybe because it breaks the idea that your mom is an innocent girl. Or maybe your brain feeds on the embarrassment with anything regarding your mother. For me, I never saw the "BBC cuck" race thing an appeal in this scenario.

Speaking from the pov of the one to do the humiliating, it depends on the situation.

If her brats are still young, and still sucking her tits almost as often as I do?
I'm cumming on her fat tits, specifically aiming to have her fat nipples dripping with my own milk. Even if she cleans them after, in her mind they'll never truly be clean. They'll always be stained by the feel, smell, and taste of my cum, and she'll always see her tits as my fuck jugs first and foremost, her family's milk producers a distant second, feeling perverse every time she uses them for their natural purpose. It will ruin the most important bonding experience that can exist between a mother and her son or daughter, and knowing I've broken that relationship before it even began, will turn me on enough to be ready to go again as soon I'm done smearing my cum all over her tits.

Reminds me of an old fantasy of mine. Not related to cum though. It is said that some women enjoy nipple stimulation and breast play and massage and some even reach orgasm from it. My fantasy is me going to trigger that part of her brain that enjoys this. So, from now on, every time her boobs are touched and sucked she would feel horny. She would consider herself a dirty whore when she breastfeeds her son and feel excited from it.

Another case of mom following her girl. Daughter Pinky June who mostly did lesbian porn, and mom Janet Darling who did just three scenes.

https://www.iafd.com/person.rme/perfid=pinkyjune/gender=f/pinky-june.htm
https://www.iafd.com/person.rme/perfid=janetdarling/gender=f/janet-darling.htm

Here are three generations of porn stars in the Vaughn family: Granny Vikki, mother Veronica, and stepdaughter Cherry. Mom seems to be the more active, and granny just did few scene, while step daughter did one film with them.

https://www.iafd.com/person.rme/perfid=vickivaughn/gender=f/vikki-vaughn.htm
https://www.iafd.com/person.rme/perfid=veronicavaughn/gender=f/veronica-vaughn.htm
https://www.iafd.com/person.rme/perfid=cherryvaughn/gender=f/cherry-vaughn.htm

Like what I say about every hot mom "It's a shame she's not doing porn".