I wouldn't recommend the smaller breeds like Shih-tzus with young children. You'd have a lot of grooming tasks with that breed, too. And, as you say, you're kept busy enought with your family.
For the same reason, I'd advise against shelties (we actually own 2 shelties).
Would you consider talking to the people at a local animal shelter & asking if they have a sturdy, medium-sized, short-haired, good natured mixed breed dog available for adoption? There's been some wonderful 'mutt' dogs that have attained the honour of Dog of the Day. And some of them have been described as great family pets.