I may not be able to tell you who got the accounts there, but I can possibly explain the two accounts with one question, did you suffer through a power surge in your area throughout the week?
If YES: The surge duplicated an account set up by a child. PS3 accounts are real picky during weather if there is a surge, and if one occurred, it would dupe the most recently made account.
If NO: Both accounts were simply set up by the children, but be safe and ask anyways. On a PS3, you have to log in with a primary account to play single player games, and since GT5(I know that you said 6, but work with me here) is user locked, both children simply wanted a go with everything in the game without racing each other. Your saves are unaffected most likely.