The parent needs to request an IEP meeting for amendment of the IEP. It would be entered as an accommodation for the child. Now, it could go one of 2 ways... either A) the IEP team will offer an aide on the bus to assure the child's safety, or B) the parent will be allowed on the bus. I'm betting on the aide but it's possible they would just make an exception to the rule for this mother. Either option would fix the problem.
My 9yo had issues with transitioning to school and back to the car every day, so we have an aide that meets us in the morning and the teacher brings him out at the end of the day to me. My 7yo has an aide that does the same thing when I drop him off. It is in each child's IEP that due to safety issues they require this accommodation.

The issue for the IEP team to put out there is that you feel they are not providing adequate care during the transition period from home to school. State your case, provide the parent as an alternative, wait for their response. If unresolved, you will have no choice but to force them into compliance.
You could also try contacting your Area Education Agency and seeing if an advocate from there will accompany you to the IEP meeting to help you accomplish what you need.

What about looking at other options? Call an IEP, discuss your concerns. If the IEP team will not help you, and a resolution cannot be reached, they will be in noncompliance. Which means it will go higher up. If no resolution there, call your civil rights office and see what they can do for you. They sometimes mediate education related issues, and you don't have to have an attorney.

Hmm, forgot to mention 7yo was on the bus and had an aide that rode with him. Same thing, in there as accommodation under the ADA.

The problem here is that the school system is only mandated to have an aid if need be. You also have to remember the busses are usually run by a bus company and not the school. This brings up issues because the school cannot control everything that is done when it comes to who and who cannot ride the bus. Specifically you are dealing with a liability issue when the person is not either a student or faculty. You might then say I will sign a waiver saying I will take responsibility if something happens to me and will not hold the responsible.

I can tell you they will not allow that because everyone in this world today is so sue happy that they would still try to sue, and a lawyer would find their way around the waiver and that is why it will not likely happen.