OP, she doesn't know anything, obviously, about child development, and that sucks. Also, people who don't live with little children have less tolerance for crying. It's natural. I think some of her comments are natural for people her age. Can you try to find some empathy for her, or say to yourself "This isn't about my mother passing judgment necessarily, it's about her wish not to be around the kids crying."
Maybe, just maybe, in her awkward, left-hand way she is trying to empathize with you or she thinks she is -- her thought process might be "my poor daughter has to deal with this crying all day long, how awful." She doesn't get that it is your children she is talking about and you feel defensive for them and it's not appropriate to comment on their behavior to you.
Maybe you can say something neutral like "I hear you, Mom, it will be nice when they are a little older."