Without being able to see or feel this “lump,” I can only run down a list of likely possibilities for you.
The most important point is to decide whether this lump is worrisome or not. Any lesion that is painful or changing (growing) needs attention. Masses that seem bothersome or that limit the use of a leg or a hip also need attention. Masses that are “hard” (feel rock-like or bony) or seem stuck to the surfaces around them are less common and merit attention.
That said, many lumps in children found on the groin (where leg and torso meet) are slightly spongy to the touch, are usually not bigger than one centimeter or 1/2 inch, and can be moved around under the skin. These are superficial masses, located in the layers between the skin and underlying muscle. The most common mass of this sort is a lymph node. Lymph nodes are found in chains all over the body and serve to drain infections from a nearby site. The chain of nodes found in the groin (on both sides) enlarge with infections in the diaper area or leg. They feel like large “peas.” Other possibilities are cysts within the skin or fat layers or tumors (usually benign) in those same layers. Point this out to your doctor and hopefully you will get a definitive answer.