Prison is great but things can get precarious if they start dying and you're down to only 3 kids in your entire dynasty. A few more deaths and it's game over. Note that they don't cause Decadence if they're in a court not of your Dynasty so if you can manage it that would be a good place to stow some of them. Also, I don't think it was mentioned but there's a global modifier based on how small your demesne is as well. With a huge Empire I was forced to get my demesne size down to 2 or 3 to take advantage of that modifier.