Hi Julia and welcome.
If by 'books' you mean published books, there is a review forum where you can review a book yourself or join in the discussion that someone else has started.
If you mean members' writing, there are various groups and you can join up to three. But you know this, as you've joined the children's writing group. Some people mark their work for group members only to see but if they haven't you can leave a critique if you wish.
Hope this helps.