Help!... Child asking questions !

Laura
Hi, could do with a little advice please . I overheard today my very nearly 7 year old boy and my best friends 8 yr old girl spelling the word S.E.X... I jumped up listening (playing in the next room) playing mums and dads. My little boy said u just throw a special seed then u have the baby. Friends daughter said no that's not how it happens. At this point I ran in and told them to play in here changing the subject Completely... He has not said anything since but I know this question is coming. He is a very young nearly 7 but she is an old 8 yr old.... I know it's too young to tell but is there anything that I can say. To put him off for a few years ( or a lot of years )... He is my oldest so never thought of this yet !... Help please x