The Harry Potter star shed her clothes for a bedroom scene with Redmayne in the first instalment of the BBC drama on Sunday, and was also seen simulating a sex act on him.
Broadcasting watchdog Ofcom has received complaints about the show, which was broadcast after the 9pm watershed, and media groups are calling for the BBC to prevent children viewing the drama online.
Vivienne Pattison, from Mediawatch UK, tells the Daily Star, "It is all too easy for them to get hold of it on (online) BBC iPlayer if they want to.
"All they have to do is tick a box to say they're 16 and they're away. We are concerned about children's access to TV programmes on the internet.
"It's not enough to just put a warning at the start of a programme and make sure it is after the watershed."
Birdsong concludes next Sunday. Here are some pictures: