That the child victim died through accident is the common defence. The best defence in any trial is always " confess and avoid"; admit that everything in the prosecution case is true but say the necessary mental element, in this case the intent to kill or cause gbh ,is not proved.
Here it's not a problem that no trace of the girl was found on the tyres or in the vehicle; forensic examination is never perfect and it's manifestly unlikely that anything would be found on the tyres.. Everything else can be admitted but the problem is that there is some evidence suggestive of the body being dismembered; suggestive, but not conclusive. But, in the end , circumstantial evidence including evidence of any abnormal sexual interest , when taken in totality does tend to swing the case against the defence. If only they didn't have to call the defendant, the case would be better than it is