Aways come down to their height level when you talk to them so you can show them and read their facial non verbal language, you want to be on their same eye level but not on their face. Don’t stare or make too intense eye contact.
Pick a topic and see how much enthusiasm the kid shows, that can be a useful clue to see if you re going the right way with your interaction. Take into account that some kids (and some adults too!) don’t like interacting at all and that is also fine.
Some popular ones are: Dinosaurs, pets, tv shows/videogames, favorite color, favorite ice cream flavor, favorite song, favorite book.