Ladies, it's good to finally feel that I'm not alone! 
As I outlined in my recent ramblings about DSS8, i have been battling with this myself, too.
DSS has only begun to independently toilet in the last 12 months - I know children develop at different rates, but really!
I can totally relate to the concept of DCs being viewed as possessions by one (or both) their separated parents; nearly three years after the divorce, DSC's needs are not top of their parents (particularly their mothers) priorities 
DP has quite recently begun to see his role as equipping DSC with the skills to lead a rewarding life, making a positive contribution to society, and he's changed from a DisneyDad to someone who can apply tough love when necessary and I love him all the more for it! Their mother seems to have a very different agenda - and manipulates and controls them to achieve whatever it is she wants (I haven't worked out what her goal is yet, though!).
There are so many complicated psychological issues at play it's impossible to know how to unpick it - childhood experiences, social pressure, personality disorders.....all are influencing factors, I'm sure!
I do know that I have certain lines that if ever crossed, would end my relationship with DP - I will never, ever share a bed with DSS (he co-sleeps at his mums), I will not get involved in DSS personal care (washing, dressing, toileting) and if his manners don't improve as he gets older, I will not take responsibility for DSS in public 
As for the rest, we'll muddle through - but I fear that there is going to be a generation of incapable, entitled adults in a few years time!