I am not sure if there are any disc sets for bmx, though you may find a racing bike or two that offers that option.

For freestyle, there is no way in the world that disc brakes are appropriate because of how often people land on the side of their wheels and would trash the disc.

Now, that said, the better option is to simply make your brakes work and ask about what you need to do and provide details on the type of brake setup that you currently have.

The best brakes in the bmx world if you run a dry bike is to use chrome plated rims, with teflon lined cables and 990 or Evolver brakes and some decent brake pads. You will stop on a dime as long as you keep the cables well lubed.