Well, when it's all PAID ticket and you want to switch around the seat (provided it happens upon boarding and no continuous switches), it's a common practice as long as the chief purser is informed at the start. I've seen people changing seat with their parents (inclusive of myself) between J-Y cabin.

Although I don't see any violation of T&C (because you are actually flying on that plane), it may look a bit obvious to the cabin crew about your intention, and at worst case, they may ask his two children to sit up front.