I have a 90 lbs (8 year old) boxer and an 19 month old son. I can understand you concerns. I was concerned not only about size but also the age of my dog (maybe not being very patient with my son). They are fantastic together. As the others have said, the dog should be more mindfull of the baby. Thor will walk all over myself, hubby, and our 8 year old son. When it comes to the baby he is very gentle and in most cases stays completely out of Cameron's way. Have you tried putting Zoey and Mercedes down together? If not try it, you will be able to judge Mercedes reactions to Zoey. I agree with Karen, try not to let Zoey take advantage of Merceds (I would be afaid of a nip from having her fur pulled). Good luck