
IMO, although I don't have any experience with Shih Tzus, as long as he is exposed to them, socialised etc, like he has been from day 1 and doesn't have any bad experiences with them then it should be ok.
My grandmother used to keep poodles and although as they got older, were more grumpy, they were tolerant of kids as I used to stay there a lot and she used to have all the neighbourhood kids in.
All dogs are capable of being snappy and bad tempered, some breeds being worse than others and of course they are moulded to a certain extent of the experiences in life that they encounter.
But I would say, just keep doing what you're doing with him. Nobody can say for certain but it sounds like you're giving it the best possible shot ;)