First, it's a good idea to show your children what these plants look like (you can look up photos online) so they know to avoid them. After exploring a woody area where these plants abound, be sure to wash any exposed body parts well in soap and water and all clothes and gear in hot water and detergent, since urushiol, the plant oil that causes the rash, can stay active and cause a rash even long after the original exposure.
If your child comes into contact with poison ivy, oak, or sumac and a rash develops, especially for the first time, you should let your pediatrician check it out to see how severe it is. Usually an over-the-counter antihistamine or hydrocortisone cream is enough to help with itch while the rash runs its course (it typically clears in about two weeks). You can also try other home remedies to help soothe your child and prevent scratching, like applying a cool compress to the affected areas, soaking in an oatmeal bath or, with your pediatrician's okay, giving your child a dose of an antihistamine like Benadryl, which will also make your child drowsy and help him sleep better. In more severe cases, a prescription topical steroid cream or even oral steroids may be needed to clear a bad rash.