Being the OP, read the original post and I will tell you that we got the third seat and it was miles well spent. The seat-belt sign was on for almost 3 straight hours on an NRT-EWR flight. He sat in 8A ( window ) and my wife in 8B ( aisle ). It was secluded enough that my wife was able to change him in his seat. Don't say anything, but she was even able to change him with the seat-belt sign on without any of the FAs noticing that she got out of her seat - not that I would recommend doing this.

Our son was two when we brought him home, (which means we needed a third seat regardless), but he was a young two and was quite interested in the plane and all of the features of it. Also, he wanted to eat when when we ate.

Speaking of eating, what would you do if the seat-belt sign came on in the middle of the main course?

In conclusion, our philosophy was we were flying in J for the comfort. If we were worried about costs too much to get him a seat, we might as well have just gotten a row of Y+ seats. In fact, I can pretty-much guarantee you that three Y+ seats is far better (comfort wise) than 2 J seats. YMMV