Hard to say, you can look at it as something she is doing for her grandson rather than for you. Lots of times people do things for someone and it just happens to benefit others around them. Money always causes such stickiness and gives someone else power over you. On the other hand, you have proof of what her intentions are, and why, so she can't come back later and complain. But, if you are in dispute with child support she could turn around and say "now that you no longer have to make payments to me you have more money" as a reason for you not pushing it with the ex.
on the other hand--don't look a gift horse in the mouth...???
I guess it depends on your immediate financial picture, if it is fairly dire then let it go, and have your situation improve, if you can afford it and it makes you feel like you are doing the right thing then keep paying or stash the money away for your child.