You cannot get it from a toilet seat, but there are many ways.
Just like the story today about the teacher who used a single sharp to have a class draw a drop of blood.
HIV is a blood born illness facilitated by the exchange of bodily fluids. It is highly unlikely that a child would get HIV from receiving immunizations or from the dentist since they use sterile instruments.
I heard of a girl who got herpes from kissing a toilet seat.