This is a very sad situation and my heart goes out to you -- as I'm sure everyone's does here. But there is a very serious potential side-effect here which I think you already realize that you need to address right away, regardless of how 'sorry' we're all feeling for the poor Pug. Coddling him right now is the worst thing you can do. Trust does need to be re-established, but not on human terms, on dog terms.
I've had experience with this with my JRT being aggressive towards other people and it's a heartwrenching thing to have to go through, so I feel for you. What I read in books and the advice I got from trainers helped me a great deal though, perhaps it can help you too. And the good news is that the solution is very humane and can be a great bonding experience.
Firstly, if your pug has not been to obedience training, now is a good time to start.
If he's already been to obedience training then he should be instructed by you to to either sit, or down (and stay) for 10 minutes at a time after each aggressive episode.
He will need to re-learn that he is the lowest rung on the pack-chain in your human home, and that he is 'safe' in submissive postures with your family. He also needs to remember that you're the boss and you call the shots.
During the 'sit' or the 'down', try not to give him undue attention and be very calm (but firm) about it. If you can tell him 'down' and he obeys then keep him like that for a few minutes. if he won't obey, then put the training leash back on him and tether him into a "time-out" so that he realizes he must obey until you give the release command.
Also, priveleges like sleeping on the couch or in bed with you should be curbed until the behaviour ceases. These sorts of things tend to give the dog a false sense of seniority in your pack if he perceives that he's sleeping at the same level (space) as the pack-leader (you). For example. They can be re-introduced once the behaviour is back to an acceptible level, if you wish.
Also, try 'reintroducing' your child to him under your supervision. Give your child a tasty treat and have your pup assume the "meeting" position (sit) before your child offers the greeting and the treat to the puppy. As long as you're watchful for signs of aggression, you can also build trust this way again between him and your child. Have your 2-year old pet him while you're there too.
Beyond that... I'm sure that there are a lot of experienced Pug trainers out here who can help you further. Agression is not a happy or easy thing to have to deal with, and I wish you all the best of luck with this. Contacting a trainer for even a few "personal" sessions one-on-one may also be of help to you tremendously if you find that the behaviour continues despite your best effort. Training and exercising your ALPHA status in a gentle way is the only real key towards eventual success that I know of with this kind of behavoural modification...
~ Daphne ~
a.k.a., Mylo and Mackenzie's mom!