Which isn't a great idea if they haven't had COVID. I do understand waiting if had it recently. Best advice there that seems to be shared is discuss with your doctor, as everyone's risks do vary.
If not though, there is a big difference seen in hospitalizations between just primary series and those who also had the booster.
I see it in my family too, sister had the booster, brother in law didn't. Her symptoms fit in the wouldn't bother testing category as they could have easily been due to chronic issues flaring a bit. He is fine, but had symptoms that certainly equated to being sick and was the reason everyone in the family did test. Oldest nephew - didn't even get a positive test, he had his 2nd dose recently so it would have been close to the max level of protection available from 2. Youngest, no vaccines, did test positive. Like most healthy kids though his symptoms were minimal, didn't appear sick.