I was also wondering whether you would have to stick around or give your name or anything once the police show up. I'm scared of retribution from crazy angry people.
It may vary by location but for the times I have called (dad holding a toddler while driving and a mom holding a 3ish month old in the front seat with an unrestrained 4ish year old in the back) they always start by asking if it's an emergency and then my name, my number and if I've called before (I always feel like a crazy lady when I say yes, LoL). I've never been called back btw.
In both cases I was told to stop following the car after a certain while and told that the police will deal with it from there. So in my case, I honestly never know if they actually do go and find these people. I always hope so, but I don't know.
But when I do call I make sure that I have the license plate, make and model of the car, color and a description of both the person driving and the child(ren). I don't know what all they need but they always take the information down.
So in the future, yup, just call 911! They will determine what action needs to happen!
And to the OP: Like someone already said, just call 911 next time! You might feel like a crazy lady at first... but the feeling wears off a little the next time you call