Mrs. laser and I did the same thing with our child several years ago when our child was about 6 months old. We took a trip to Europe with our child in our lap. And yes, we had to pay 10% of the full fare for our child, not 10% of the price we paid for our discounted tickets. The rules are quite clear on this.

We did take full advantage of the bassinet attached to the bulkhead as our child basically slept the whole way, much to the relief of everyone around us. Also, the FAs were great and often stopped by to check on us and our child. So, I have no complaints about paying the 10% of full fare for a lap child and think it should remain that way.