Why aren't my child's hands being affected by poison oak or poison ivy?

Split question?? Your question can be interpreted several ways and other answers cover the basics. We often see the palm side of the hand or fingers not break out when the rest of the body or back of the hand will. Some suggest the thickened skin of the palm (or soles of the feet) is resistant to the chemical that triggers the rash. That has been my personal experience with pi.
Not all folks get it. Only about 80% of people are sensitive to the irritant effects of urushiol, the chemical in the resin of these plants that binds to the skin and causes that darn, itchy rash so many of us know. That means your kids are part of the "lucky 20" - the 20% or so of folks who can roll around in poison ivy or oak and walk away with nary an itch.
Allergy takes time. No one reacts to poison ivy or poison oak with the very first exposure. This requires repeated exposure over time. To develop a skin allergy one must become sensitized to the irritant known as an allergen. Thinner skin seems more prone to react. Yet everyone reacts on a different time table. That is why many claim not to be allergic to poison ivy--they have just not reached their magic number yet!