I hadn't considered this issue at all - thank goodness I checked in on the Lonely Planet forum and saw a related discussion!
I keep forgetting that others don't see us, the single parent & kid, as we do. I keep forgetting that there are assurances to be given that I haven't taken my kid along with me illegally. It's one of the biggest hassles of cross-border travel! When we flew to Australia recently to visit my parents, I had to get all sorts of signed and stamped affadavits regarding custody of my son, to prove I hadn't kidnapped him or was taking him out of the country illegally.
Trouble is, this is something that's never gone to court. I haven't seen my son's father since I was 3 months pregnant, and it's only recently I've started to hear from him. I don't have any legal documents giving me sole custody - only what I can come up with in sworn statements.
But it's something I'm going to have to look into. I need to ensure I won't have hassles at a border crossing with regard to the kid.
Perhaps this should finally go to court - once and for all.