Help! Risk Assessment for Farm Park trip

Hi all
Looking for some advice. We are planning to take the pre-school to the local farm park (typical one with indoor play centre too). Its only 10 mins by car and so the committee have decided that instead of hiring a coach etc then the parents will be responsible for transport there and back. The parents are organising it between themselves to transport each others children. Do we need a risk assessment for this and what do we include?
Once at the farm then either the parent stays with their child or the staff take over for those that are there without parents. We have asked the farm to provide us with their risks assessments what do we need to do ourselves?
Has anyone done a trip like this before? Any advice? Things we should consider?
Taking first aid, mobile, address book, register. accident forms etc.
Im new to this so trying to get as much info as I can! Thanks