Solution:
Have kids. Have a couple. You know what kids do?
They barf. They get fevers. They cough, and they have boogers. Un-sick ones. Constantly. Have a couple and you'll immediate be an assay for sick-kid/not-sick-kid. No other tests needed, just "Parent Eye." An attending told me this in residency and I thought it was the stupidest thing I'd ever heard.
But he was right.
You'll also learn how murderous it is to have a kid who's "not sick" torturing you all night by screaming and coughing and vomiting all over your carpet all night while not being "sick." You'll realize, hauling off to the circus environment of the ED at 4 am is only half as painful as staying home with a sick kid. You'll never look at those 3am moms with the same scorn again.
I know that doesn't help you right now, but here's a shortcut just as good, probably better. You wanna know how sick a kid is? Just ask...That's right. Jut ask the mom. ("Yeah, yeah, I've heard that before. Doesn't work, not as good as a decade of schooling). Ask. She'll tell you. Put all your Peds EM, PICU rotation stuff aside for a minute and ask. Just sit there. Look at the kid for a little while. Look at the mom. Try to absorb how worried she is. And ask, "I know I'm the doctor and you brought your son/daughter here to be checked out by me, but if you had to be the one to decide, 'How sick would you say Bobby is? How worried are you?'"
90% of the time you'll get your answer. Then you put that information through you Peds EM filter machine and come up with a plan. It works. Never blow off mom-stinct (mothers intuition) especially if an experienced mom. Trust me. I'm not a mom, but I live with one. They know.