In my experience, placing an observer into a stressful non-routine event will yield questionable results. In the last five years, I have spent probably months in the ICU (for work). Highly stressful work, hundreds of alarms and people literally dying every day. The difference is for those who work there, it is, for a lack of a better word, routine. And even in that case, it takes at least a day for them to ignore me. Hawthorne effect, Heisenberg effect, whatever you want to call it. That first day of observation rarely, if ever, offers any good nuggets.
But this problem is travel with children under 5. Hardly a routine, just as stressful and is a very short duration. Adding another person will add another level of stress. The additional person will never disappear and I think is likely going to have a significant effect on the experience.
I do like the OP’s notion of going to a train or bus terminal and observing. I didn’t realize this project was so exploratory. I wouldn’t recommend either an interview or survey in this case. This is literally throwing spaghetti at the wall. Once the OP sees what sticks, that will provide the direction for any future interview or survey.
And I’d like to read the paper on the “unstructured” interview. If there is a link, please post it.