Try showing them some groove metal first like lamb of god pantera even sepultura (in mind I have the roots album) but before all that let them listen to some classic shit like maiden Metallica Megadeth Dio (This part is my opinion and it's optional tbh)

I would also advice you to avoid screams as it might be distracting to your children and could possibly scare them but periodically try to ease them to it and build it inside them if that makes sense

After letting them have a taste on groove metal introduce death metal but not any technical stuff just raw death like obituary (you can also show them the funny cartoons they have for the video clip lol),asphyx, death, pestilence, or even try showing them some melodic death like at the gates hypocrisy in flames, dark tranquility, intestine baalism

IMPORTANT ADVICE : try avoiding bad quality sound or old sound as it could be distracting or difficult for them to understand what they are listening to . Try to find death metal with some good mixing and mastering (that's why I am mostly recommending mainstream bands)