The scenario in which an AirTag in a car seat would help prevent a child abduction is extremely specific: the child would have to already be in the car seat and

The scenario in which an AirTag in a car seat would help prevent a child abduction is extremely specific: the child would have to already be in the car seat and not subsequently moved therefrom, and one or more of the kidnappers would need to have their iPhones on (since the tag uses the network connections of nearby iDevices), in service range, and either not trigger or ignore anti-stalking alerts. I can kinda see how those requirements might fit a custodial kidnapping, but only if the partner didn’t know about the tags. Stranger kidnappings (which are very rare anyway) would likely not. A GPS tracker with a cell connection would serve you much better in either scenario. Those are less popular with criminals because they can often be traced back to the purchaser, but they are more reliable and won’t accidentally alert the kidnappers because they tripped an anti-stalking monitor.