If they could date and the child not know then that would work. If you are asking if a teacher can date a student who is a parent, then no. A teacher cannot date any student.
Is it legal to date your student?
In many countries, it is illegal for a teacher to date or have any sexual contact with a student, no matter their age. Professors are required to alert the university should a relationship develop between them and a student to ensure no unfair advantage is given to the student.
Is it illegal to date a student who is 18?
Child Molestation and Student Teacher Relationship Laws Sex with students over 18 may not be a state felony, but there are laws protecting minors from sexual abuse at school. In the state of California, any sexual contact with a student under the age of 18 is explicitly prohibited by law.