i thought the same thing as you (cant think of a username)
i booked my holiday first, then my sister wanted to book her's after but the places werent available on the system even though it said there was.
i tried this for several dates ranging from may-july and nothing for the both of us.
the are quite a few rooms left at our hotel both s/c rooms and b&b rooms but none with a free childs place after i had booked.
its all very confusing isnt it.
you flight supplement is quite alot- i only paid 2 flight supplements @ £25.00 each total £50.00
i would have a word with your travel agent
the only child cost i have had to pay is his meal which is £8.00
i hope my travel agent wasnt tell me porkies as its my friend from school