To me their only real con is the difficulty housetraining them. They are small and require a little more attention than the average pet, but they are so worth it. I have never seen a dog that is so loving! They are almost like having a human baby the way they like to cuddle and be fawned on. I suppose that could become a con instead of a pro, if you did not have the time to give them all the attention they demand. They are very manipulative. I admit that mine can get me to do almost anything they want. I'll be busy paying bills, cleaning house, whatever....and here they come with a toy they lay down in front of me. First time it is very civil, but if I ignore them, they can get a lot more aggressive with their demands of time.
I usually give in and get down on the floor to play for a while before they yelp or use their nose to give me the shin nudge.
Another con that some may not like dealing with is grooming. You can minimize this by keeping them cut short but one of the great things about a Yorkie is their beautiful coat so I still have mine long. I am thinking of going short so I can use belly bands though. I just hate to cut Ben. His hair is so long and pretty! But it takes a lot of time. If I skip a few days grooming, he looks terrible and I feel guilty.
They are very intelligent, have personality plus, and are great companions. Mine know when I do not feel well and seem to give me extra attention in a more gentle way than normal. One actually will curl up on my abdomen where I hurt.
They do tend to bark a lot when someone approaches our house. Although it is a bit annoying, I do not want to discourage that. I am often alone and feel like no one could ever sneak up on us here. Still, it happens so often, I worry that I might ignore them, much as the "Little Boy Who Cried Wolf" got no attention when he really needed it.
If you have plenty of time to spend on training, playing and loving on them -- I think Yorkies are a wonderful pet decision! Your daughter will probably love dressing one up and most love to be carried around a bit. You can even get a pet stroller to help. Read on here about finding your Yorkie. You want to find a good reputable breeder to make sure you are getting a quality pet that will have few health or behavior problems. Stick with AKC registered for better percentages. Good luck!