He does not have to prove his innocence - the Crown have to prove his guilt. He has entered no plea at this stage because quite possibly his solicitor does not have the full file yet. He will have to enter a plea at the PCMH (Plea and Case Management Hearing) at the Crown Court. If he pleads not guilty, it will go to trial. If he pleads guilty he will stand convicted and will be sentenced.
Levels 3, 4 and 5 are the most serious levels of child porn and depending on his previous, his character and the number of images could easily result in a custodial. In any event, he will be placed on the Sex Offenders Register and will be unable to work with children. He may even have conditions imposed about association with children.
I just hope your step daughter sees sense.