Good luck!
I'm in the process of making a complaint against CAFCASS so have not had a good experience.
My advice? Keep calm, be yourself and keep note of everything that CAFCASS say and promise. Also, provide them with documentary evidence (after the meeting if necessary) to evidence any concerns that you have.
If ex is going to use a charm offensive, there is nothing that you can do about that but you CAN provide CAFCASS with evidence that he's actually a lying, untrustworthy, emotionally abusive tw*t. (In my experience, CAFCASS will seemingly ignore such facts you place in front of them but they're not supposed to, hence the advice I've received that I have to raise a complaint...)
Anyway, the most reassuring advice I received was :- whatever happens, if it results in an order that you think is not in your child's best interests (e.g. because they have to spend more time with an arsehole who doesn't actually care about them or because it gives him more opportunities to be abusive to you - which is not in your child's interest either, you can appeal against the court's order or apply for it to be varied. And if CAFCASS make recommendations based on disputed facts (ie if they accept ex's lies instead of checking actual facts presented to them by you), they can be challenged on this.
But please don't go in there assuming the worst just because of my experience - you might get lucky and have a decent CAFCASS person who sees through your ex. And, if you don't, there are things that you can do to challenge their incompetence.
Good luck!