There is no law against covert recordings. It's best not to tell social workers that they are being recorded. You are more likely to catch them out being verbally abusive, aggressive or dishonest with you that way.

I would have waited those 3 years to have a baby with him and abstained from pregnancy until he was off the register.

You need to start digitally recording everything. If the social workers are lying, catch them out. Don't let them know you are recording, either.

I would go ahead and do it anyway, just don't tell her. You won't be punished for recording her. If she doesn't want to be recorded she has something to hide and you could really catch her out.

Make sure you bug the children when they go in to speak to the social workers. Plant mobile phones on them set to record and make sure the social workers are not aware of this. You will have everything the social workers are saying to your children. IT IS VITAL THAT YOU DO THIS. Good luck.

You forgot to remind people to RECORD. Record, record, record. I cannot say this enough. Recording is easily done in secret without the knowledge of professionals. This is the best way because corrupt professionals will let their guard down and they will think they can get away with it if they don't...

Really I would get rid of him. Having an affair is one thing but putting your family at risk by soliciting a minor is the ultimate betrayal. I wouldn't stop him from seeing his kids, but what he has done what with putting his family at risk is rotten. He will probably be put on SOR and your children...

You need to record every interaction you have with the social workers. If you can catch them contradicting themselves/lying then it could help you in court. You don't have to let them know that they are being recorded. This way it's best because they are likely to let things slip when they don't know.

How did children's services get involved in the first place? Who told them that you are together and that he has a conviction? This information would be helpful to me if you want any advice.

I honestly wouldn't put it past them. I have heard personal accounts that would suggest that they do and can access people's servers. Put a VPN up on your computer (virtual private network) and make a new facebook account. Create the email for the account at tutanota.com where all emails are encrypt...