Sometimes people request an Uber or Lyft and when the driver arrives they have a baby and no car seat. If this happens to you just say no.
They can object and scream and yell as much as they please, your answer should still be no.
Any child who is below the age/weight to sit without a car seat is a risk for you. They can’t sit on the adult’s knee, next to them, etc.
It doesn’t matter if the passenger says they’ve done it tons of times in Ubers and Lyfts before – don’t let their carelessness be your liability.
Car seat laws vary by state, but every state has a law and you should find out yours so you can authoritatively tell the reckless passenger it’s illegal.