I'm sorry, but your DM is as entitled to have a moan as anyone else.
So what if she has been a SAHM ? So what if she only works 3 hrs a week ? Why on earth should that mean that she cannot have a moan?
By that way of thinking, if you get migraines, then no-one can ever moan to you that they've got a headache ?? Grow up.
She made her own life decisions and has lived quite happily raising her family. She still has regular commitments to her DM, DH and her DC, which she does on top of the usual cooking, shopping, cleaning etc so her life IS busy.
If she does not want to fill her days with childminding her DGC, meaning zero time to herself after she's also fulfilled her other family commitments, so what ?? She's DONE her child raising, and is now on the next phase of her life.
You and your DS have also made your own life choices. It's up to you and your partners to find solutions if you think life is too hard right now, not your DM.
Would YOU be happy for your own DC to start telling you that your life is too easy, therefore you should absolutely never moan to them, because they've got it so much worse !!