The issue here is the "official line" is different to how it works in reality! The only way to GUARANTEE seats together (whether you have kids or not) is to pre-book your seats at £35 each way per person. The reality is, they don't seat kids away from their parents, but it is possible that could happen. It happened to us on Virgin. Even though we had contacted their special assistance department 8 months before we left and again 3 months before we left (because our DD who was 8 at the time is autistic) on our return leg from MCO to LGW, they sat her in a different row next to a stranger adult and wouldn't let one of us switch with her until after take off!
So to answer your question, it is very unlikely that they will sit your children away from you, but it can happen and the only way to absolutely guarantee that doesn't happen is to pay to pre-book.