What can I do if my mother won't let me parent and takes control in all things concerning my child? She even becomes angry when I assert myself as his mother and I disagree which later escalates too far. I just need to let my mom know in a respectful way I am their mother.

It sounds like your mother is not letting you take an active role as a mom to your children. The situation also seems to give off the idea that your mom thinks you are incompetent as a mother. Do you know where her need for control over her grandchildren comes from? Have you done something to make her think you are perhaps incompetent as a mother? I can see that it is frustrating that your mother is affecting your ability to be a mom, even if she has good intentions. However, it is also important to set boundaries, because as your children's mother, only you would know what is best for them. Your mother is more than welcomed to give advice, but her behavior seems to replace your role as their mother.