I see the problem. In English we would say my Mom doesn't (does not) agreee.
I have an idea - you can PROMISE your Mom that each day you will wake up early to take the dog out. Take the dog out means - take the dog outside so the dog can go to the bathroom. You can PROMISE your Mom as soon as you come home from school, you take the dog out for a walk. Maybe then she will be willing to let you have a little dog.
They are very cute.