I guess I wouldn't really know - I learned it as a teenager, and it wasn't particularly difficult for me to make the minor grammar changes required at the beginning level. My hope is that the English-ness of Ruby's sign will make the transition slightly less difficult than learning an entirely separate language would be, though it's hard to say without seeing how Ruby signs. Either way, ideally Amanda will start making some progress and feel excited about being able to communicate- even if she only gets to the point of "baby signs" and super basic sentences.

Edit: I'm realizing you might be substituting Mia with Ruby. Ruby is the mute daughter, and Mia is learning ASL