identical skin discolor on each of my sons abdomens

When my first son was age 2, we noticed several cafe-au-lait spots on several differnent areas of his body. All different sizes, from a thumb print size to covering one side of his whole abdomen.  His doctor wanted to rule out Neurofibromatosis.  Our son went to an opthomologist and had his iris and optic nerve checked out as ok, then we saw a dermatologist that said he had what was called a flag pattern on his abdomen.  The cafe-au-lait spots were a sign of neurofibromatosis but this was the only symptoms our son had. If there were a problem,we should see other signs appear by age 5.  He is 5 now with no new spots and no new signs.  We have noticed this week that our 2 year old son which also has about 3 cafe-au-lait spots, now has the same dark pattern on his abdomen.  Can you explain.  They both have red hair if that matters.