Its very unlikely that the barbs will spawn in a breeding trap (it would be pointless just to put one female in a breeding trap as any eggs she lays will be unfertile if there aren't any males present at the time, and they're unlikely to want to spawn in confined space like a breeding trap), your best bet is to plant some very fine densely leaved plants in the tank (for the barbs to lay their eggs in), create the right conditions for the barbs to spawn, and then remove the barbs from the tank after they've spawned and then raise the fry once they hatch. If you keep the adult barbs in the same tank as the eggs are in, they will eat any fry that hatch from them.
One thing you need to consider though is if you actually have the space and the time to raise the fry, how large is this mini aquarium exactly- (edit: i just saw your sig, the barbs are in the 15gal yes?) is it even really large enough for the adult barbs in the long term? Raising fry can be very time consuming as they need to be feed a lot more regularly than their adult versions, and can take a long time until they are large enough to be rehomed or sold etc- when i breed my pearl danio's some years back, i ended up raising the danio fry for almost a year.
