Be under no misconception, the CAFCASS report will have a huge impact on the judge's decision. So you need to think really carefully about how to tackle this.
You could ask Dave what his position is within CAFCASS. How qualified is he to make such decisions? Has he discussed it with his supervisor. You could also ask for info regarding their procedures and how they are meant to handle your case (ie isn't he meant to meet you all first, I'm pretty sure he is and in specific circumstances, but often this is ignored).
You are entitled to ask these things, you are entitled to ask for information and help. But asking questions like this will get you labeled as a trouble maker and may well put Dave's back up. But it is the only way to get a suitable qualified person in charge of your case and to make CAFCASS sit up and take notice and (hopefully) deal with your case properly (don't hold your breath on that one). However deciding to go in with guns blazing at the start isn't always a good idea. If that first report that goes to the judge says you have been hostile, negative and obstructive then you are stuffed basically. It will take many months to prove otherwise, but then it's going to take many months to sort this all out anyhow.
It is absolutely true that in court you need to appear reasonable. And in fact you should actually be reasonable at all times and be putting your children first at all times. It's very important for you to come across as doing this at all times.
Otherwise your ex's opinion of you as the cause of any problems is more likely to given by Dave as the reason for the problems and this well be seen by the judge (who will rely on Dave CAFCASS and the few minutes they see you in court to make some really important decisions about your children.
Dave knows this and to threaten you with putting this in the report (and it is a veiled threat in reality) is completely unacceptable. He is basically making a judgement on you without having met you yet or allowing you to discuss things with him. I would tell him that if it were me. But you may decide to play it a bit more softly at this stage, which I would also understand.
Find out all you can about CAFCASS. And no I don't mean their cosy 'aren't we great' website. Though you should read that thoroughly so you can use things they say ion there to back yourself up.
www.cafcass.gov.uk/default.aspx
Have you checked the OFSTED report for your local CAFCASS? You can do this online. If it's rubbish not good, like the one in my area, you can use that and say you wouldn't use a child minder with such a poor OFSTEAD report and until you meet him the OFSTEAD report is all you have to go on. Course they may have a brilliant OFSTEAD report, but somehow I doubt it. I look forward to being proved wrong on that one day though!
You could perhaps say to him that once he has met your whole family and heard everyones opinions and points of view that then you would be more happy to consider his suggestion. That you want him to meet the children when they are calm and settled so he can see what a difference the situation is when their dad arrived.
But say that right now you don't believe his 'one solution fits all' approach is appropriate and that doesn't he think it is reasonable for you to meet him and have your say too before you would consider allowing a stranger to make this kind of decision about your children.
Ask him if it's unreasonable to put your dc's welfare and emotional welbeing first, because that is all you are trying to do. And remind him and make it very clear to him that you are simply a mother trying to protect her children from the emotional abuse they have been subjected too, which has been witnessed by others.
If you don't agree with the arrangement he is pressurising you into then offer an alternative. Ask for supervised visits at a contact centre for now. And agree that this will change if Dave, the contact centre staff and everyone involved there think it's appropriate to do so. Also ask for mediation between you and youe Ex if its not already ahppened. You will need to show you have tried this in court too, so best to jump in with the offer early if you can.
Ask Dave to work with you not against you and (if you feel this way) perhaps tell him that right now you don't believe he is working for you and most importantly your children and you believe a meeting is the first step. Ask him to confirm what their proceudre is because I'm you're pretty certain that their procedure is to to meet everyone initially and discuss the children and the issues around contact before making decisions like this...
Word everything in such a way is that you are trying to get his help here and looking for assistance to help all of you work through these issues.
Ask him exactly what will happen should there be an issue with the children at handover.
Not trying to scare you sorry but I have visions of my neighbour's screaming children being torn from her arms by several policeman and being forecably put into the father's car kicking and screaming because the court order had to be followed. Don't ever allow yourself to be the one who looks unreasonable, but don't sit by and let CAFCASS, Dave or whoever, flout their procedures and policies. They're not the ones who are left picking up the pieces years later. In fact most of them have no clue what the results of their decisions turn out to be in the long term.