www.gosh.nhs.uk/parents-and-visitors/advice-when-you-stay/preparing-treatment
www.bch.nhs.uk/story/whats-it/getting-ready/having-operation/talking-your-child-about-operation
Both of these give excellent advice that I used in helping prepare my daughter for her surgery.
Things I found most useful were:
Ideally before you declare that you're going to hospital play with dolls/teddies drs and nurses, get the teddies to do the same thugs she's likely to be asked, take temps under arm, take medicine, use a stethoscope, display affected area, lie still etc.
When you at doctoring teddy be sure to ask consent and explain what and why you are doing. Drs should always do this but often don't with small children.
Always finish with something like "and the dr says you can go home now" rather that all better as they may well still feel rough.
Notice period = 1 day for every year of their life.
When you teach this point encourage them to pack a suitcase ready with what they think they need, add things you think she needs. Pack and unpack, role play arriving and leaving. I used to say we are going to the pretend hospital and when it was time to pack the case for real my language changed.
At the hospital don't be afraid to ask the drs to explain again if she doesn't understand and really try to get their consent for examinations or procedures. I did this with my 2.5 year old much to the amazement of the staff.
If you do have to override lack of consent explain that you hear that she doesn't want to but you know that we need to let the drs do x to get better.
Sorry that so long, I spent a LOT of time researching this last year and am eternity living it on the ward.
It's worse for you than it is for then xxxxxxx