It is the oil that causes the rash. That's why scratching spreads the rash like wildfire because you're just spreading the oil from the leaves all over your skin when you scratch.
The best thing you can do is to make sure they're wearing protective clothing, including pants tucked into tall socks and long sleeve shirts tucked into pants (good preventative against ticks and leeches too). They also need to know how to recognize poison ivy and they need to know not to scratch if they come in contact with it. You should carry some dish-soap with you since it's the best at cutting through oils and some calamine lotion to soothe the leftover rash after the affected area has been thoroughly washed with the soap. They should also wear bicycle shorts under their pants if possible in case their pants come into contact with it and they have to take them off and bag them up to avoid spreading the oils around to someone's skin.
I don't know about preventatives other than the clothing, but more power to you if you can find one. An ounce of prevention is worth a pound of cure!