She is in pain and on pain medication.
There really is only three options at this point.
1. Surgery to cut out as much cancer as they can but it will come back, they aren't sure how long maybe 3 months maybe a year.
2. Let it go and eventually she will be in to much pain to walk to her food bowl or go outside to the bathroom.
3. Put her to sleep.
You have two children that were born after you had the dog who love her and will be heartbroken when she dies.