Internet suggests a median height and weight is 30cm and 7.5kg. I have no idea what that means but would expect him to about double in size, but they are one of the smallest "toy sized" breeds.
Wow. He's going to be very small indeed. Are you planning to get another, bigger dog when your children grow up? Since Moshi is a therapy for the eldest, he might be able to learn more from a bigger, more powerful dog once the son is old enough.
Yeah they are a small breed, but surprisingly one of the oldest -- I believe as far back as 400 AD in the Chinese court; modern pugs are quite a bit different as they've been bred for specific traits (like cuteness!). I would love a big dog, I never really wanted a little one but we don't have the space for anything else at the moment, and the wife definitely does not want a big dog. Maybe when we have our own house, with a large area of land, then I'll get my big dog and it can have the run of the place. :) My neighbour has a Belgian Shepherd (~ 60cm, ~30kg) and they get on very well, but that thing is like a bear! Here I just snapped this for you, it's like a Where's Moshi? puzzle: