I am so sorry to hear that this is happening to your children. Definitely call the bus transportation office, and let them know that the children involved have been bullied and harrassed by this 2 kids. Call the school, make an appt. with the principal and the guidance counselor, to set up a meeting regarding this situation becoming way more serious than it already has. I know you mentioned that one of the kid's mom doesn't care, and it wouldn't hurt to have her come in too and hear from your children and this little girl along with her parent's, what these 2 bullies are doing. This child's mother may need a wake up call and realize that maybe this 11 year old is a victim too of the 15 year old putting her up to the bullying. I could be wrong about this, but, importantly, your children and the little girl have rights as well as safety, to get back and forth to school without being harrassed or bullied. There used to be a program several years ago, where students themselves would become Hall Monitors, also before/after school monitors, who would try settling problems between the students involved, and it had seemed to work out well.
In the meantime, your children and this little girl need to feel safe to and from school, and the bus transportation office and the school administration, need to be made aware of the fact that they are being harrassed and threatened. Good luck!!!!