This is a bit of a mess as it is my experience and investigation of some of your issues.
The only way you can defend accusations of violence is by proving you weren't in the vicinity at the dates the alleged incidents occurred. You will need to find witnesses who are happy to come to court and testify this if they give you a statement, plus as much documentary evidence as you can find. Use your commonsense when choosing witnesses - so no mutual friends or his family members as they will be unpredictable on cross examination.
You need to get the case numbers asap, find out what dates the alleged incidents occurred and get your written defence statement with some supporting documentation in to the court quickly.
If your ex and his solicitor has any intelligence they would also allege your molestation of him was also non-violent as this is far harder to prove against if he has any evidence such as messages. It takes only a couple of unpleasant messages to an individual to be accused of harassing them.
Apart from getting legal aid, which tonnes of people are not legible for, a non-molestation order or undertaking means that you cannot go to mediation to sort out issues. This actually may be more advantageous than you think as it means you both cannot screw around and drag it out. In the case of children, judges have to rule in their best interests. You know your ex will be arguing that both children should be together mainly with him, but as they have an age gap of 6 years you could argue something completely different so at least the child with you mainly stays with you.
There is no point going to the police or social services about your one year old unless you have serious claims about how he can't care for them so they are at risk. Unfortunately in your case you should have done this immediately he didn't return the child as in the eyes of the law he is an equal parent to you so equally capable of looking after the child.