I know it seems crazy, but I wonder if Celebrity’s “system” is incorrectly considering Children pricing as a discount that cannot be combined with any other additional disc count. Since the website clearly shows both prices with the discount Celebrity is screwed up. Did you try calling them to book the excursions? Although the detailed conditions say it is a “web” special, I know they will book the special pricing on the phone. And also the small prints says you cannot cancel and rebook, but that is also not rue.
I know you said you called, but did you just ask the question about why you cannot book children pricing in line? If yes, I suggest you call and actually book the excursion on the phone.
Good luck.