Unfortunately in-cabin sitting is not an option on NCL. I do believe they charge a lesser amount for children in the speciality restaurants, I cannot recall the details but I know we considered taking our 3 and 5 year olds with us and had discussed the possibility with the restaurant host in Shogun. Which to be honest would have been a complete waste of money for our kids. They would never eat any of the stuff on the speciality restaurants menus, and I didn't think to ask if they could order from the kids regular menu.
As it turned out, the kids loved the kids corner offerings on the buffet and we would get them dinner about 6:00 p.m. at the buffet and they could eat and then at 7:00 we would let them spend a few hours at the kids club while d/h and I enjoyed a wonderful, romantic dinner in the speciality restaurants. It worked well for us.
NCL's kid's club's accept children as young as 2 and I can tell you the kids love it. Mine are always upset when we come to pick them up at the end of the evening. They will also give you a pager in case they need to reach you.
It's strange on conventional cruises without freestyle dining our kids love getting dressed up for formal nights and eating with the grown-ups. But they also love eating their kid food and getting to go play, so whatever you choose to do, it sounds like a win-win situation