How do i file a complaint against a urgent care?

Good choice on your part, since fluid imbalances for a child that age develop quickly. You can talk to a local personal injury attorney about any action. It may be that there are no damages. You can also talk to the attorney about how to file a report with state licensing agencies. Some caution is required so that you do not overstep. In particular, I would be concerned about their failure to summon an ambulance after they said they would and you relied on that statement.

You asked "how do I handle this?" - that depends on what outcome you are looking for. If you want to tell the owners of the facility about how you were treated, you should certainly write and tell them - they may care about making sure that this doesn't happen to anyone else. Unlike hospital emergency rooms, urgent care facilities do not have to care for every person that arrives on their premises. But if it was a facility that has contracted to care for those who have the insurance which covers your grandson, you can certainly let that health plan know how you were treated. It sounds like the communication on whether or not they would see your grandson / what they would do wasn't great but may not have caused harm to him. If that's the case - no harm - there is little to no chance of a a successful civil action for damages. There is no licensing agency for urgent care centers in California (or most states for that matter.) The "license" involved is that of the physician(s) who run them. Hope this helps you understand at least some of your options and that your grandson is doing well.