i think you should definately cash it. She obviously wanted you to have it or she wouldn't have sent it.
People get a lot of pleasure from giving. I don't think you should deny her that pleasure.
Maybe, becuase of the situation, it's been hard for her to stay in touch with you and your son throughout the year.
Christmas is the ideal opportunity for her to let you know that she still thinks of you.
She could be hurt if you just don't cash it.
Maybe you could send a thankyou, explaning how you feel, just as you have done here.
could you say that you were very grateful for the kind gift, but you understand that she may have other commitments in the future?
Perhaps open a savings account for your son?
I must say, if I were in her position, I would want to let you know that I still care for you. After all you split up from your partner, not his family. x