I am sure that Childline are moderating adequately to ensure that adults or other children are not able to contact the children outside of the forum. I am also sure that all content is moderated and that some is not permitted onto the forum.
However it is still a public forum where children are discussing sexual abuse, masturbation and their sexual feelings and experiences, which any adult can read and any adult can pretend to be a child, sign up and respond to as long as their message passes moderation. The NSPCC/Childline can not be so naive as to think that there will not be some predatory men who are doing just that and actually some of the messages read very much like they come from adults.
Some of the advice from children (or adults) to other children is wrong and/or unsafe and Childline do not appear to intervene to ensure that the correct advice is given. Some of the advice, or lack of proper advice, is potentially dangerous. Even if Childline were giving proper advice behind the scenes or advising the posters to make contact on the phone, other children who look at the boards would be misinformed about an issue that they may be concerned about.
Another issue is that, whilst there are lots of replies on some topics, on others there are often no replies at all. Some children describe a rape or sexually abusive situation that they have been subjected to and then noone replies. How on earth would that feel to that child to think that no one cares and again even if Childline contact the child via a message direct to their locker, what message does it send to any other child searching the board to find a situation similar to his or hers and then discovering one but without any replies or even an acknowledgement?