Parent's request to hold back the child in same class.

As per section 16 of RTE Act, no child admitted in a school shall be held back in any class or expelled from school till the completion of elementary education.”

It binds the school though not the parent directly, but the outcome applies to the child irrespecive of who requests detention. The detention is illegal. 

Even if the parent request that the child be detained , the school authority cannot do so. The consent of parent does not override the stuatory prohibition. 

Promotion up to class 8 is automatic by law and not merit based.  You may receive thre request made by the parent in writing but politely refuse to entertain the same, you may state that  Promotion up to Class 8 is automatic by law not merit-based.

You may state that as per Section 16 of the RTE Act, detention in elementary classes is not legally permissible.”

Yes.  A parent cannot request to hold their son back in a class and as per Sec.16, it is not permissible legally.

Child cannot be held back under section 16 of RTE till amendement in 2019, which specifically provides for 

The child could be held back only in accordance with the provisions of Section 16 (as amended) of the RTE Act notwithstanding the wishes or whims of the parent.

“16. (1) There shall be a regular examination in the fifth class and in the eighth
class at the end of every academic year.

(2) If a child fails in the examination referred to in sub-section (1), he shall be
given additional instruction and granted opportunity for re-examination within a period
of two months from the date of declaration of the result.

(3) The appropriate Government may allow schools to hold back a child in the
fifth class or in the eighth class or in both classes, in such manner and subject to such
conditions as may be prescribed, if he fails in the re-examination referred to in
sub-section (2):

Provided that the appropriate Government may decide not to hold back a child in
any class till the completion of elementary education.

(4) No child shall be expelled from a school till the completion of elementary
education.”.

This is not a question to be answered by lawyers.  Strictly under law the child cannot be held back in the same class. If the child is held back the next question would be for "how many years and will the child be ever fit for the higher calss?" It is necessary to ask here why the parents want to detain their child. There are mentally retarded children in this world. They will be in need of special care in special schools. Blindly following the law will do them damage.