As a mom, it is possible she may be embarrassed by her son's neglect of his child. Sending you some money could be her way of assuaging that guilt. She probably shouldn't feel guilty - it is his debt, not hers. But as many of us have come to learn, feeling our children are a reflection of our parenting is often the "norm" and it can take years of alanon to outgrow that feeling.
Since the money is a gift, perhaps you can just accept it as a gift... not as some sort of replacement "child support", but just as a gift from a loving grandmother. And sending a thank you note for the gift would not be out of line.
I wish you well. ((hugs))