I Have A 12 Yr. Old Grandson & A 8

I have a 12 yr. old grandson & a 8 yr old grandson who caught by the 8yr olds mother showing each other their private areas and the 12 yr old apparently laid on top of the 8 yr old (both boys) ...when ask what & why the 12 yr old stated he was showing him on man & women do it. This is the 2nd time this has happened, but the last time was quite a while ago. Should I be concerned.