Bring it to the teachers attention and impress upon her how serious it is and she can get the child seen by the guidence counselor or the school pyschiatrist. At that point, they will have to talk to the mother as well, but at least the school is informed and maybe they can help the mother get counseling for the child as it seems as if the child needs it.
Taking a 6 year old to a funeral is not something I'd do at ALL. Especially another child's funeral. Sounds like the mother isn't all there either.
But I think you are right in wanting to raise the warning flag.
Please let us know what happens.