a lot of the squre taper bb kits are road bike specific, they do fit bmx but the taper will be short as road bike cranks tend to be thinner at the boss, try to find a bmx specific one which should have a longer taper
just about all os ally cranks with square tapers will always work loose , the crank boss will deform over time, especially if you weigh more than your average 6 year old , hence tha reason for big kids ally cranks being made a lot fatter, using pinch bolts etc , once the wobble starts , the cranks are starting to deform, you can get more life out of them by using shims on the taper, but this will not stop the deformation , or if you can, try to find a bb with a longer taper
