Yes, but that seat which the child has must be very close by. In an emergency (such as strong air turbulence up ahead), they may say everyone get seat belted in your own seat immediately, and your wife cannot take you son back to your seat during that time because everyone has to remain in their seat in their cabin.

So that's the reason this cannot be done without their being an empty seat next to your wife, because it's very unsafe. Do you want your kid to die or get seriously injured because they're thrown against the ceiling in air turbulence because weren't seat belted? Is that risk worth this crazy scheme you're considering?