This page of the HSE has some great information on Farm Safety & how to keep the children safe on trips -
HSE Campaigns Safe Farms
Some of the pdfs are for the farm admin's point of view but excellent awareness from a trip organiser's view as well ..please be aware that there is also some pretty hard hitting information on there.
Fantastic PDF, for KS2 perhaps -
Stay Safe - printouts & activities
H & S guidance & publications from Teachernet -
Educational Vists
The Handbook for Leaders looks good
Safe Farm Trips !
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