Apparently she is OK legally:
RSMO 566.032 defines First Degree Statutory Rape.
1. A person commits the crime of statutory rape in the first degree if he has sexual intercourse with another person who is less than fourteen years old. 2. Statutory rape in the first degree is a felony for which the authorized term of imprisonment is life imprisonment or a term of years not less than five years, unless in the course thereof the actor inflicts serious physical injury on any person, displays a deadly weapon or dangerous instrument in a threatening manner, subjects the victim to sexual intercourse or deviate sexual intercourse with more than one person, or the victim is less than twelve years of age in which case the authorized term of imprisonment is life imprisonment or a term of years not less than ten years.
Comment: Missouri only prosecutes statutory rape if the older sex partner is a man because statutory rape was originally designed to control teenage pregnancy. This new statute seeks to curb the sexual exploitation of any young person, which could signal an end to female immunity from prosecution.
RSMO 566.034 defines Second Degree Statutory Rape.
1. A person commits the crime of statutory rape in the second degree if being twenty-one years of age or older, he has sexual intercourse with another person who is less than seventeen years of age. 2. Statutory rape in the second degree is a class C felony.
Comment: In English: after your 21st birthday, if you have sex with a person who has not yet turned 17 years old, you have committed Statutory Rape. If you have not yet turned 21, your sex partner must be at least 14 years old to avoid breaking this law.
As you can see from the law cites above, since they were both 14 or over, there is no law broken (assuming it was consensual sex).
In my opinion you are doing right by keeping them apart. They are clearly too young to deal with the responsibility of a child. When he turns 18, if they are still in love, then its up to them.