Hi WTBH, I don't understand your lawyer. The therapist has reported, the children have been interviewed twice, you have found inappropriate photos on DDs iPod. Surely this is enough to get an interim order for supervised access pending further investigation? What if you took the photos directly to the police? They would be required to take action.
Have you considered contacting an organisation that advocates for children who have been abused. Sorry but I'm not from your country, but I'm sure there would be one that could advise you. I understand you're not sure if it's happening, but if you have good grounds for suspicion, there may be some action you can take for the immediate future.
Thinking of you. Stay strong.