Here's some information from Keep Kids Healthy:
Quickly wash any areas of your skin that has come into contact with poison ivy. You have about ten minutes to remove the oil before a reaction will develop. Also wash any clothing that has come into contact with the plants to prevent spreading the oil and scrub under your child's fingernails.
For mild reactions, you can use calamine lotion, hydrocortizone cream, an oral antihistamine, oatmeal baths and cool soaks.
More severe reactions will require evaluation by your doctor, who can prescribe an oral steroid or a stronger topical steroid cream. Oral steroids will need to be taken and slowly tapered over two weeks to prevent recurrence of the reaction.
Good luck.
Peggy