I honestly can't remember if there is a gate but the field is down a bit away from the road. I don't think it's completely secure but if your children are old enough to understand how far they can go it should be ok. If you are really worried then camp at the bottom of the field. It's quite big so you would catch a toddler running before they got to the road.
Just to warn you the loo's are very very basic. They don't flush it just washes the paper down the hole iyswim. You will be fine across the road. It's normally where we try to go. There is a lovely walk at White Horse Hill and in the evening they sell logs so you can have an open fire which is fab for marshmallows! I like it but it's a field with a basic loo. I hold my breath!
.....the loos! Going with friends who are a little more particular than me. Are the portaloos of the flushing kind? And if we were to pitch across the road near the farmhouse is it still safe for the kids with the road next to us?