I'm sorry you lost all your fish.
The only good thing is that you get to start with a clean slate now. I would keep the barb, if your son likes it, and up the school to about six or seven. I have seven in my 29gal semi-aggressive community and they look really cool. Here's what I did with my son (who is almost 7) when I set up the 29gal in his room. I took him to Petsmart and Petco and let him show me what fish he liked. Then I went home and we researched the fish on the internet to see what would work and what wouldn't and how many we could have. Then we went back to whatever store had what we had decided on and bought the fish. Be sure to add the fish a few at a time, not all at once, or you'll overload the cycle you have established now and lose more fish. It may take a little longer than just buying what you like and tossing them in the tank, but you'll have a happier, healthier, prettier tank for it.
The only good thing is that you get to start with a clean slate now. I would keep the barb, if your son likes it, and up the school to about six or seven. I have seven in my 29gal semi-aggressive community and they look really cool. Here's what I did with my son (who is almost 7) when I set up the 29gal in his room. I took him to Petsmart and Petco and let him show me what fish he liked. Then I went home and we researched the fish on the internet to see what would work and what wouldn't and how many we could have. Then we went back to whatever store had what we had decided on and bought the fish. Be sure to add the fish a few at a time, not all at once, or you'll overload the cycle you have established now and lose more fish. It may take a little longer than just buying what you like and tossing them in the tank, but you'll have a happier, healthier, prettier tank for it.