Dan as a professional maker of women crying (NEVER intentional, ok, maybe 15%), I assure you the best thing to do is NOT panic. The first 2 dozen times or so you want to panic. But you just be reassuring (say it's ok, don't worry, etc..), and NEVER EVER scream the words WHAT ARE YOU CRYING ABOUT NOW?. Just some felpful tips for all you guys out there....
NEVER SAY: "it's not a big deal" or "stop crying." or "i'll give you a reason to cry." girls hate that last one especially.
My dad would always say "just stop it" and we'd yell and scream and tell him we couldn't.