Can we ride with 7 if 4 are kids?
The Guru Take
Most likely not - Seat belt laws
You must be referring to UberSUV, UberXL, or Lyft Plus that hold up to 6 passengers. The short answer is "no", even if 4 are kids, since the car may not be equipped with enough seat belts, you would most likely not be allowed to ride legally.
The seat belt laws are different from state to state (in USA), so you may be okay with the seat belt situation. However, whether you will be able to ride will be up to the driver's discretion who needs to be aware of the terms they have signed with Uber/Lyft as well as the insurance they carry.
The similar is true when a larger car shows up that has a 7-passenger capacity (such as a Suburban or Escalade. In that scenario, it will be again per driver discretion whether he or she will let you ride. (If you are a driver, you may want to be mindful that Uber and Lyft may frown upon on this and you should be mindful of potential effects on insurance in case of an accident.)