Kids don't need ID BUT...dd and I have different last names (kept my name when I got married). Leaving New Orleans once on NW, check in agent gave me a terribly hard time- Why do you have different last names? How do I know you're not trying to kidnap her?- etc. Everything ended up OK, but not before he called security and I asked for his supervisor (and I wasn't being nasty or snotty- the last thing I wanted was to have my 3 yo taken from me, into custody, 'til it could be straightened out). I always travel with dd's passport now, just in cse- and have NEVER flown NW again!