"Sexual intercourse" has a specific statutory definition under RCW 9A.44.010(1); it includes the ordinary meaning of intercourse, any penetration by an object, and oral sex. Generally speaking, the age of sexual consent in Washington is 16 years old and there is no requirement to get a parent's approval. However, communication with a minor for immoral purposes under RCW 9.68.090 is prohibited. If this is accomplished through the sending of an electronic communication, then this would elevate the offense to a Class C felony. "Immoral purpose" is undefined and a rather vague concept, but the 20 year old in this scenario needs to be aware of the potential consequences.
No, parental approval is not necessary. However, you should be aware that communicating with a minor for immoral purposes is a crime, even when actual sexual contact isn't.