A child ticket is the same price as an adult ticket.
The website won't allow you to select 0 adults, 1 child.
Therefore you have two options. You can either buy the ticket over the phone, which costs �1/minute, or you could just book the ticket as an adult ticket on the Internet.
I wouldn't have thought there would be any problem with the latter option, but I haven't tried turning up with an adult booking and a child.