Also, you only have to observe the child for moments. Have a notebook and pen as if you're comparing prices or something. No one questions folks writing in stories, I always have a list I'm scribbling on and sometimes that scribbling has nothing to do with shopping
But no one ever looks at me funny.
Make a quick glace at a child doing something.
Turn to your notebook -- what caught your attention in that quick glance?
Now listen...what kinds of conversation is the child engaged in. My experience is they tend to be talking most of the time to SOMEONE. You don't have to look at all for that one...just make notes.
Now look past the child at stuff on the shelf near it with a slight frown as if you're really concentrating...you can still see the kid, what's it doing? Climbing the shelves? Waiting patiently beside the mom's cart with one hand on the metal edge? (If it is...check to see if it's secretly a robot)
Quick glances with lots of distracted frowns and scribbling on your notes can get you a huge amount of data without alarming the mom in the slightest. Everyone in stores seem to be trying to remember stuff.
But I don't really recommend just sitting down and studying a kid. Sometimes you can get away with it if you're really grandmotherly but most of the time, folks will look at you funny. But since you're instructor would like you to make the description pretty short in time duration, you don't need to do too much settling in as you observe and record.
I find stores work, parks work (since you can look at lots of kids and not seem to interested in one...often folks assume the person plopped on the bench in the shade belongs to one of the kids.) -- the library can work but libraries can be kind of paranoid too.