It's true that not many murderers plead guilty but the reason is more complex than not getting a reduction in sentence for pleading guilty.
Many of those charged with murder hope to get a verdict of manslaughter, on the grounds of diminished responsibility (basically circumstances amounting to such provocation that they lost self-control). That's because so many murders are domestic, and the accused will point to behaviour by their wife or lover which put them 'over the edge'. This defence isn't as easy to run as it once was, when 'crime passionel', anger at infidelity, was often enough but now isn't.
Otherwise, they run 'self-defence' or 'no intention to inflict grievous bodily harm' thus denying an element in the offence.
This man,really alleged to be a paedophile may try any of the paedophile defences: 'accident', that is he either didn't assault the child but she fell in the river, or some such accident (!), or that he did assault her but not with the intent of harming her badly, or of course, that it wasn't him seen picking her up. The second one has an unpleasant, but neat air to it, because it's admitting everything apart from the intent. The best criminals admit everything they can but deny intent or dishonesty, the mental element in their crime.