Beyond telling DD the EP will be there to work out how DD could be better supported, I wouldn’t prepare her. That sounds harsh, but preparing DD is likely to give a false impression, particularly if she is less anxious after being prepared.
The EP should speak to you, but parents aren’t often present for the whole assessment in school. I would request to speak to the EP beforehand to ask questions such as what assessments will be undertaken, what observations will be done in addition to the 1:1 conversation (e.g. structured and unstructured times, with peers, with adults). Ask if the EP will make recommendations for the need for further assessments, e.g. OT, SALT where the LA believes they are necessary. Also, ask the EP to ensure the report is detailed, specified and quantified. Finish by requesting the report is sent to you at the same time as it is sent to the LA - you may receive push back on this, but if you request it, it should be done.
It is helpful to make a list of anything you would like to say and a timeline of events. Think about things like: strengths and needs, what is working well, what isn’t working, what has been tried but hasn’t worked, what is needed but unable to be delivered, DD’s likes/dislikes/what makes her happy, what is important to DD/you and what is important for DD, how DD communicates/how others communicate with her, what she wants to do in the future/aspirations, a bit about family/any other important people in DD’s life, important history/developmental history.
If DD is unable to attend school that day, the EP could visit DD at home or, if DD could manage it, she could just go in the meet the EP. Although if there is going to be a lesson observation, that would be preferable wherever possible.