It’s a little different with kids. For kids your most concerned that their is a congenital heart defect or myocarditis.My first son has a really low resting heart rate. As a first time parent I was freaked out by this. Our pediatrician said she didn’t see a cause for concern but I had her refer us to a specialist twice to check it out (once when he was born and once after we had a hospital stay for RSV and the damn alarms kept going off due to his heart rate).
Had him checked and both times the doctors said sometimes people just naturally have lower heart rates because the heart doesn’t need to pump as much to get blood through the circulatory system. All the tests and everything were fine and it was nothing to be concerned about, FWIW.
I want to stress this point because people seem to be glossing it over: many people have a low RHR and a low RHR itself is nothing to be concerned about. However when that low RHR starts causing problems (such as fainting) that’s actually something you need to really stay on top of and is now something that needs to be regularly monitored.
As a parallel to make it easier to understand: snoring itself isn’t a problem, many people who snore do not suffer from sleep apnea. However if that snoring develops into OSA (obstructive sleep apnea) then now you have an issue that you need to monitor.