First of all: Stay calm. It's extremely rare for people to go to the doctor to late just due to "not caring enough" unless it's old (mostly rural)folks. (One of the first things I learned: When a farmer calls an ambulance, always take full ALS gear with you) Or caused by mental health issues or financial constraints. People are far more resilient than we generally think.

Then: Most industrial nations have "medical helplines". In Australia it's 1800 022 222, in Canada 811, in Germany 116 117, etc. Resesarch your local number and if unsure: Call them.

Then: Look at the so called ABCDE Scheme.(Extended version of blood goes round and round and air goes in and out and any deviation from that is bad)

Airway: Anything that fucks up an profoundly airway for more than 30 seconds is an issue - call an ambulance. Aspiration, foreign body obstruction, anaphylaxis reaction with airway issues. Extremly runny noses (as in RSV) and associated breathing problems warrant an ED visit. (But seriously people, get your kids vaccinated)

B: Breathing: Anything that continuously makes breathing problems is a "go to the ED" or "call an ambulance" thing. Continuously (!) is the point. It's normal for someone to have a coughing fit or breath a bit heavy when having a flu. But there is a difference between "my lungs are gonna kill me, i need to stay on the sofa and watch netflix" to "breathing has become so bad I actually have to focus on it and one flight of stairs slowly would make me feel woozy" to "okay,now I really really need to fight to breath enough ".The second one is a reason for an urgent care visit, the later one for an ambulance call. Also: Look for the lips and the area around it. Does it look blueish? If yes: Seek help. There are countless examples online. With children it's a bit more difficult, to be fair. But as a parent you often will know when - when you manage to stay calm. Signs of acute need to seek help: Children whose chest kind of "cave in" between the ribs need an ambulance. Children who can normally focus on you or other things and don't due to being focused on breathing? Call an ambulance. Children who are having audible breathing problems (as in: you hear them in a quiet room and it's not their nose) will need an ED visit. And again: If they become blueish/whiteish. But again: There is scientific proof that parents who manage to remain calm and get a calm observant look on their kid identify urgent and critical cases better than healthcare professionals. The staying calm part is hard,though.

Circulation: In adults: For fuck sake people: If you have chest pain that is not triggered by a certain action (e.g. a pain to the wall of the chest when breathing in deeply, a slight pain when coughing, etc.) call an ambulance. And especially for women: Strange abdominal pain, neck, arm or jaw pain counts. Especially when paired with shortness of breath, when it gets worse when you exert yourself. Or when it stays more than a few hours. Or is paired with very low or high blood pressure relative to your normal blood pressure. If you feel something pulsating in spots where normally nothing should be pulsating maybe see urgent care. Previously unknown dizziness when standing up? ED.

With children again it's a bit more difficult. The good news is: They very very rarely are compromised circulation wise,they hold themselves together for a long time (and then crash). But: It takes a lot for the latter to happen. Generally: Massive and sustained vomiting or diarrhoea are an indication to go to the ED, sooner than you would do as an adult and the smaller the earlier. A very good indicator is the recap test, look it up online.

D is "disability" in this scheme and meant in the sense of "neurological issues". They are actually easier than most people think. It's obvious that an unconscious person should get an ambulance, as well as a seizure (please also call for febrile seizures). If someone is showing neurological deficits by either being disoriented, absent or having sensory or paralysis-like issues don't wait,call an ambulance. If someone is suddenly vomiting uncontrollably and having a headache or any other neurological issues: Get to the ED. Likely a migraine,but there is a slight chance for a very very bad other reason. (And migraines aren't fun either).

In children look up meningism signs - can happen due to fever as well, but that's a good reason to go to the Urgent care clinic.

E is meant here as exposure,but covers bleeding,trauma, abdominal issues and infection as well. Seek help for scaldings/burns and any bleeding that you can't stop within minutes or that requires more than a large bandaid. Seek help for anything that does belong in the body. Sustained abdominal pain that makes a child unable to be calmed down for more than 1 hour is a very good reason to go to the ED. A bladder infection in a child is a good reason to go to urgent care. An abdomen that gets hard as a brick when a little bit of pressure is applied is a reason to call an ambulance.

Fever is a bit of a hit and miss situation, especially in children: First: Fever and sepsis are NOT the same. You can have a bad sepsis and have zero fever. (The last guy I nearly lost to fever had 34.5°C and never went beyond 36.5° before). Second: Fever sadly has the issue of causing febrile seizures and putting a lot of strain on the circulatory system. Which is bad. Third: But a bit of fever is nothing bad per se and there is more and more scientific evidence that an too aggressive approach to reducing fever is a bad idea as well in children. So...in the end it's a bit of a question of moderation. Give something when the child is actively "sick" and unable to do most things due to that. Give something when the fever goes beyond 39.5° C. Fever itself is not a reason to go to ED or call an ambulance - the symptoms that go along with it might. (And please get a proper thermometer and not one of these "forehead" or touchless ones. And don't try the house remedies of lowering fever like putting cold wet towels on the patient...they have all been proven to make it worse)

This is just a little bit of advice. And you don't know if I really know what I am talking about. So please read up yourself. Get a first aid course and a children's first aid course. Check local resources and where urgent care options are. The ED and ambulances are the worst options - both in treatment quality, resources and often comfort.