Those are very cheap brake calipers that are probably flexing under braking. The easiest way to get significantly better braking is to replace them with more modern calipers. Tektro is one such brand that makes inexpensive but robust calipers, in a variety of lengths.

If you really want to go with discs, consider replacing the fork for one with disc tabs. Keep in mind that you'd need new wheels (or at least new disc-compatible hubs and new spokes laced to the existing rim).