When kids get sick

It's a conundrum. When do you take the kids to the doctor?
I never solved the puzzle when my children were growing up and I can't help a lot now with our grandchildren.
It's always tough because kids seem to get sick on holidays, weekends and just before you're leaving on a big trip.
It's difficult to decide how sick they are, whether they'll get better on their own and when expensive and time-consuming medical intervention is warranted.
The only real rules seem to be if you wait too long, the doctors scold you for delaying and if you go in too soon, the doctors scoff at your over-zealousness.
It's very frustrating.
I once went about six weeks before I took my baby daughter in to have her arm checked. She'd broken or cracked her shoulder bone when she rolled out of the crib (with a little help from a bigger brother) onto the carpet.
She couldn't tell me what was wrong but she looked at me funny when I'd lift her little arm very high.
Weeks later, the doctor confirmed that she had a small knot on the bone where it had been broken.
"Of course, it could have happened during her birth," he said, trying to console me as I tried to deal with being a completely clueless, heartless mother.
Other times, I'd check in with an ailing, coughing child who seemed destined for the hospital only to be told it was a common cold. I ended up nearly losing a daughter who couldn't stop tossing her cookies. Several times my son's headaches were actually concussions. Another daughter reacted badly to a bee sting and sunlight and scared us silly before we had the sense to get essential help.
It's really a dark art and as I watch my daughters try to decipher the code that defines a serious ear infection or strep throat from a minor stomach flu, I agonize for them and with them.
My grandson's "stomach ache" just turned out to be appendicitis.
My 3-year-old granddaughter isn't getting over what some newby doctor diagnosed as a simple virus two weeks ago.
You'd think I'd be better at diagnosing after 40 years at this but it's actually made it harder. I know it can be silly to run in to the doc for every sniffle but I also know what can happen by waiting too long.
Is 60 too late to apply to medical school?