If you child had an iPhone 6s or newer, then you could pair a GPS only Apple Watch to the iPhone, even if the iPhone does not have a SIM. The GPS Apple Watch does not care about the iPhone's cellular carrier. Basically a GPS only Apple Watch is a fancy Bluetooth headset for the iPhone. And Bluetooth devices do not care about whether you have cellular or not. Your child could still use FaceTime audio, Walkie-Talkie, messaging, email, get the weather, track exercise activity, and of course, tell time.
But the child would need a newer iPhone. The 6s is the minimum (1 year newer than the 6).
The other possibility is to use the Apple Watch Family Plan. This requires one of the Cellular Apple Watches that support Family Plan. In Family Plan a parent's iPhone is the controlling device, and the child gets their own add-on cellular plan. The parent's iPhone cellular carrier must support Family Plan.
https://www.apple.com/newsroom/2020/09/apple-extends-the-apple-watch-experience-to-the-entire-family
https://support.apple.com/en-us/HT211768