A jury at Bradford Crown Court heard a boy aged five was forced to perform a sex act on the older boy, in return for being allowed to play on a trampoline.
The boy, who cannot be named for legal reasons, has been cleared of similar charges against an eight-year-old.
He will be placed on the Sex Offenders Register and sentenced on 28 March. The incident happened last August.
The Crown Prosecution Service said it believed the 12-year-old boy was the youngest person to be convicted of rape in West Yorkshire.
Neil Franklin, chief crown prosecutor for West Yorkshire, said: "Any case where the perpetrator and victim are both children will always involve extremely difficult decisions and very careful judgement on the part of prosecutors, particularly where there is evidence of a sexual crime.
"We are satisfied that the right decisions were taken in dealing with this case."
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