Thanks for this. There are actually prices for children that are slightly discounted on international itineraries. It seems that it is about 75% of the adult fare, but maybe this is changing, and hence it's not bookable anymore?

Rectification: after trying different options, it seems that it's only when an international itinerary includes a codeshare (in that case with Swiss) that it becomes impossible to proceed with the booking for a party that includes a child.