I reckon the ones who would point blank refuse to switch seats so a mother can sit next to her little boy just coz they don't HAVE to, are the same type who would drive a 10 mile round trip to the shops to demand 10p that the shop assistant shortchanged them. And they would demand a refund for the slightest petty little thing that they deem as wrong or unsuitable. And they would carry 25 items in their pocket and handbag before paying 5p for a bag. And they would report someone sneakily for a slight misdemeanour that most people wouldn't bother reporting, (like a fellow employee on a ciggie break that they weren't meant to be having, or someone using the internet or their mobile phone in works time.)
They probably live for opportunities to make peoples lives just a little bit more miserable. They probably rub their hands with glee when people regain the weight they lost, or if someone goes bankrupt, or has their home repossessed, or if they find out someone's husband is having an affair. They probably revel in telling kids Santa's not real. They probably pop kids footballs with knives when they get kicked over into their gardens.
They probably scoff at people who cry at sad movies, or who buy soppy cards for their partner or spouse, or who go to the beach and build sandcastles, or go to a kids film, or to Disneyland without kids. They probably laugh at anyone over the age of 15 who likes Justin Bieber or One Direction. Grumpy, miserable, negative, curmudgeons.
I could be totally wrong of course. 
And yes I would absolutely swap to let the OP sit next to her little boy. Anyone with an ounce of compassion and decency would. Thank F most people are not like some people on this thread!!!