Keep in mind, my experience with both breeds has been those owned by others (both pet and performance/show homes). Jack Russells, like most terriers in general, can be aggressive with small animals (and sometimes with larger animals, too). They are also a very active breed. If not given enough mental and physical activity, they can be destructive. They can have an "I rule the house" attitude. Because of that attitude, I'd be leery of getting one with young children in the house.
The Shih Tzus I've been around have had very nice personalities. With that breed, I think I'd be more afraid of them getting hurt by accident with a toddler in the house.
The Shih Tzus I've been around have had very nice personalities. With that breed, I think I'd be more afraid of them getting hurt by accident with a toddler in the house.