소꿉친구 stems from the word, 소꿉놀이. 소꿉놀이 is the well known universal game in which small children play the role of a father and mother. So, 소꿉친구 means a friend who you have known since you were very young. The Korean dictionary says 소꼽 is wrong and the standard word is 소꿉 but in reality, you hardly find people who say 소꿉.

소꿉친구 childhood friend