Well, generally if the parent bounds all the children, if the children get bigger, the parent sill needs to bound the children, so an update to the parent is required.
So you might
- do a pass over the objects, and update transforms. If child transforms, scale, rotate (maybe not rotate with spheres, but aabbs ) or translate, then the parent bound is potentially invalidated
- do a pass that transforms a unit bound, or recalc some how, for each object, any parents bound the updated child bounds
- do culling