Cock Crows at Eleven (1978)

A man walks in on a 12 year of girl while she's in the tub. At first she seems upset but then doesn't mind. She gets out and lets him watch her dry off. We see her breasts and side of her bottom. Then she lies down on the floor naked and lets him have his way. Full chest exposure while the girl is in and getting out of the tub. At the point of where the man is seen having sex with her, the girl is clearly replaced by an older body-double.

After having sex with an adult man in the bathroom, a 12 year old girl is dressed in her undershirt and panties. At the man's suggestion, she strips her clothes off and climbs back into the tub, where we briefly see her breasts again.