@Jack B.
How many kids do you have? If zero, try not to judge as I’m guessing you can’t actually relate although you may want to think you can...
I was pretty quick to judge before I had one. My 3 year old ranges between an angel to a terror in seconds as his undeveloped brain sees fit which is way more often than I care to admit.
Tantrums and meltdowns are so unbelievably embarrassing especially in public or during first impressions. Sounds like counting to 5 slowly was attempting to avoid the meltdown and it didn’t work. Sucks to be them in this situation.
Guessing rental references or meeting at their current residence (dropping off an application) would be a better way to judge how in control of their children they are or how much damage to suspect than a 1 off meeting. Also paying attention to how clean the kids hands and faces (not clothes necessarily) are can be a good sign of how on it the parents are. Look for the wipes. Dirty hands make a mess in a hurry if left unchecked.
That said I have only rented to 6 inherited tenants with children and it has been a mixed bag from “couldn’t tell there was a child here” to “lots of stickers and dirty fingerprints on each wall 3 feet up.” My house is holding up well to my toddler and was newly renovated before he was born.
Good luck. Just wanted to offer another perspective.