Making small chat with kids is generally easier than with adults: they advertise their interests. With the toddlers, you don’t even need to be verbal… exaggerated expressions (not scary ones, just being more expressive than you normally would, same as they are), facial gestures, looking at them and then “hiding” after the back of the seat in front of yours. I’ve had “conversations” with toddlers with whom I didn’t have a single word in common
With older kids, they’re likely to wear items related to a favorite character or series, or you get information from them or their parents about school, after-school activities or hobbies. Bandaids are also a source of conversations: my 5yo nephew seems to have one somewhere every time I see him. The last time, I gained several points’ worth on account of trimming the ungainly one on the tip of his finger to a length which was still protective without being in the way. “Do you like pink?” or “I’m not familiar with that animal on your T-shirt, what is it?” are phenomenal leading questions for the younger crowd.
The biggest point is, I think, respecting them, and that implies listening to what they have to say, asking for clarification when there is something you don’t understand (what? grownups don’t know everything!), being willing to answer their questions (up to and including the dreaded “huh, I don’t know that either”) - and letting them not-talk if they don’t want to. One of my pet peeves as a kid was grown-ups who tried to pass as if they knew everything when they absolutely did not… it shows, but IME the kids with the tons of questions prefer “I don’t know” to “because it is so.” The ones who don’t have a ton of questions are likely to be even easier: they love talking, so you just have to ahum at the right spots (just like my mother, so I have tons of practice there).