Am I in favour of people who are guilty of raping minors being executed? Absolutely.
Am I in favour of executing people convicted of raping minors? Less clear.
Rape, particularly that of a minor, tends to depend on witness testimony. Often that of the child. This may sound fine, but children make appallingly unreliable witnesses. They are extremely vulnerable to suggestion, and hazy in their recollections. Frankly, I'd support such a punishment where the evidence was absolutely unequivocal. But juries simply can't be trusted to deliver guilty verdicts ONLY where that was the case.
As an aside I think it would be more helpful if persons guilty of such an offence were subjected to mandatory psychological research so we can spot potential offenders/potentially abusive relationships better. However, that would be completely unconstitutional since it would be 'unusual'.