There are some comedians - Boyle is one of them - who take the approach that absolutely nothing is out of bounds. Some people find them funny. I remember Denis Leary used to have loads of fans who had cancer because they loved his cancer jokes. Within hours of a disaster there will be jokes. Now if you are disinclined then you just don't watch it.
FB will joke about anything. Many people will glean from this that he hates and disrespects everyone. Others will argue that his humour is detached from how he is as a person and what he actually does/believes/thinks, and in this case that his 'joke' reveals absolutely nothing about how he views or treats disabled children. Everyone's stance is their own business.
Similarly there are others who don't find him funny but don't think his act is necessarily the man. Nor might they believe that defending his right to cater to his audience makes them haters of disabled children by default; indeed the notion seems nonsensical to them.
It works the other way too. Look at Chris Langham - inoffensive comedian, looking at child porn in real life.
The KP issue itself and this business of hiding children away...I dunno, but I don't see KP as someone who has proudly shown her son to the world, rather as a fame hungry and ruthless woman who has shown the world everything, including her other children, in whatever way she thinks will successfully market herself as a brand and therefore make money.
Others will disagree with my cynicism. Fair enough.