I know another mom who wanted to get one representing herself and her daughters, so she did 3 butterflies. A large on in the middle and two small ones floating around it, and the colors were their birthstone colors. Maybe incorporate your son's birthstone colors into something that better symbolizes your boys than a butterfly probably would and that would be cool!
I would almost love to have a portrait of my children done, but it would have to be a decent size to look right and of course you ahve to find a tattoo artist who is amazing with portraits--make sure to see LOTS of their previous work!
Have fun deciding! Let us know what you pick :)