L.C.
I'd suggest backing off. Whatever snowjob the mom is doing you are certainly not going to be able to buy your way out of, nor do you have any hope of convincing her children that she is the bad guy. Major waste of effort, there.
Children are not for appreciating their parents... even if they have 12 kids of their own and live to be 100, they will *never* appreciate all of what was done for them, what it cost their parents or how long it continues, because they will always be younger, and always have a younger perspective than their (hopefully) more mature parents. Children are also not for respecting their parents (or any other presumably more mature human), they are for respecting and treating gently and kindly with understanding that they're young and don't understand.
Because they are young and don't understand.
Be yourself. Love their dad. Continue your generosity because you believe it is the right way to behave, and do not ever hold out any expectation that these behaviours will ever get you anything except a nice warm, smug feeling that we all get when we know we're being the mature ones.