iPad Stolen by Employee

Last week I fired the manager at the preschool I own and while doing inventory I see two iPads are missing. They’re not available on Find My iPad but I did mark them stolen- they wouldn’t have known the Apple ID but the pin was the same on all of the iPads. Could they have factory reset them with a new Apple ID? If so, would my marking then stolen have any effect? One of her kids thanked me for letting them use my iPad and she quickly corrected the child to say it came from someone else about two weeks prior to her termination, so pretty sure the family has both but can prove nothing. If marked stolen with my Apple ID but signed into with a different one, will they still lock when marked stolen? The mom factory reset the work laptop prior to returning it to me, wiping all the files she knew I needed (though everything ended up being saved to One Drive so NBD), so I’m very interested in making sure those iPads become paperweights if she did take them.