I'm afraid @wafflesmgee clearly has no idea how appeals work. Their advice is completely wrong. You will not win an appeal by talking about how well your child fits the admission criteria. Unless you are arguing that a mistake has been made, the admission criteria are irrelevant for appeals. If you make your case entirely about the admission criteria, you will lose as you will not have given the appeal panel any basis on which they could award your child a place. An appeal is about showing that your child will be disadvantaged if they don't go to this school and convincing the appeal panel that this outweighs any issues the school will face from having an additional pupil. It is about allowing giving a child who doesn't fit the criteria but really needs this school a chance of getting a place.
Their second piece of advice is no better. The appeal panel cannot tell the school how to organise itself. Suggesting that a TA could be moved from EYFS to KS2 will simply be ignored.
@ThatsGoingToHurt - The fact the school regularly goes over PAN in these years helps you as it suggests they can do so again without too many problems.
The fact they are under PAN in infants is helpful, but the panel has to consider the possibility that the school may fill up in those years, however unlikely that may seem. If PAN is 60 and they only have 45 in, say, Y1, if they receive applications for 15 pupils to go into Y1, they must all be admitted.
Your son's EHCP is not relevant, I'm afraid. I am guessing that you might want to argue that there will be transport and/or childcare issues if your daughter isn't admitted to the same school as him, but that is not something appeal panels can take into account.
Appeal panels generally aren't too interested in Ofsted reports or a school's reputation. However, if you can identify specific provision at this school that is missing from the offered school and will be beneficial for your daughter, that will help. The forest school is a good point in this regard, but it would be stronger if you have medical evidence to back up your daughter's need for this provision.