On the 7th episode of 'Heirs', a scene of
their past showed how Kim Tan (Lee Min Ho) and Choi Young Do (Kim Woo Bin) went
from being best friends to enemies. Viewers noticed that the child actor who
plays Lee Min Ho's child counterpart looks familiar because he apparently
played his counterpart four years ago in 'Boys Over Flowers'.
An 'Heirs' rep confirmed, "The child actor who plays Kim Tan's child counterpart is indeed Jung Chan Woo. He also acted as the younger Goo Jun Pyo, played by Lee Min Ho, in 'Boys Over Flowers'."
Did you notice when you watched episode 7?