5 month old Shih Tzu started growling at kids?

She is otherwise an awesome dog who loves me a lot. She however started growling at my daughter who is 2 while my daughter is asleep. Now today at the park when kids showered her with affection she started growling and I had to remove her quickly before she snapped. She is otherwise happy. She has been leash trained and has been doing good on potty training. What is going on why is she aggressive with kids and how do I quickly correct this before someone gets hurt?