I am concerned for the safety of MY child - i do not think that is being Unreasonable.
So what do you expect expelling an abused child would achieve. Do you think s/he will just evaporate in a puff of smoke and never pose a risk to anyone else again? As far as I can see, these are the options:
the child goes on to another school- and what about the safety of those children?
the child ends up being home-schooled, possibly by its abusers, and problems worsen- what about the safety of anyone the child comes in contact with?
the child ends up in a PRU unit- does the safety of those children not matter?
the child ends up on the streets- as above?
If this is indeed a serious and permanent problem, then wherever this child goes, unless the problem is dealt with, there will be other children potentially at risk. To those making the decision, every one of those children matters as much as your child.
Lets face it. how would you like a child with sexualised behaviour being moved into your class because it was considered unacceptable that some other mother's child would be at risk?
On the plus side, abused children can get help. Children displaying inappropriate behaviour can get help. Many grow up to function perfectly normally in society. But unless you actually propose killing them, they have to grow up somewhere.
Otoh safe-guarding needs to happen. If there are any further concerns that this is not happening, then you need to take it up with the school.