I dont understand why Sonny killed those two kids? What does the term rolled over mean? From what I understood he killed them for beating Michael and Fredo at pool; I highly doubt it!

I forget the reference in the book - I tried to forget it as quickly as possible after reading it - but the term "rolled over" usually refers to the police getting guys who are arrested, to turn state's eveidence (become "rats" or "snitches") and cooperate with the prosecution in the prosecution of others - usually higher-ups.

In other words, get them to "roll over" on their associates.