DragonLee: yeah i do love my mom very much even more so when my dad died last year she`s now the only parent i have left. i do agree with you that should say out of there grandkids situation between the parent and just be there for the kids especially if it being worked out between the parents.
Dragon, I'm not in a situation such as yours, but the way I see it..... You would not be lying to your Mom. YOU ARE SUPPORTING your children. Maybe say that you and your ex agreed to split all costs 50/50 because that works best for both of you. Or, perhaps when she asks how much child support you are paying you might try a different approach, say something like...... the support changes depending on what the kids needs are. Like I said, I'm not in your situation, but I think it might satisfy her curiosity.
My Dad is all I have left, and he's not doing all that great at the moment. He usually, no not usually, always asks me, "what'd you pay for that"? everytime he sees I have something new. I just pick a figure that I think he thinks will be reasonable and he's happy and I'm happy. Sometimes a little white lie keeps the peace and it doesn't hurt anybody. Sometimes I just want to say, "what does it matter?", but he's my Dad and I respect him and love him very much. Like you said earlier; their actions and words come from their age. They came from a different era. In their minds, they still think of us as their little girls or boys, their children, even though we now have the role of their "parent" so to speak. One of the aids that's caring for my father in the hospital at the moment said something to me the other day that really made me think. "In life, you're an adult once and a baby twice".
I just had a thought, imagine that!

Maybe the reason for her asking is because she is concerned that you're paying too much????
Hang in there! You're being the best son and father you can be. Which isn't always an easy job.

You and your Mom