Ditto on the Miata. Very nice handling car in base form; can be made very fast through the corners with the correct mods. Lots of vender support and a very large community of tuners, drivers etc. to use for car setup, driving technique etc. Parts are relatively cheap and the light weight should keep you from going through pads and tires so quickly. Not too fast (straight line) for a budding driver but will definitely allow one to learn the art of driving.
Porsche 944 - I like this car too but it's a Porsche which means replacement parts are going to be $$$. Better hope you never zing the engine or tranny. I hardly see any at the track which means (at least in this area) I wouldn't have anyone to talk to about setup, technique etc.
BMW 325 - less expensive than the Porsche and more power than the Mazda and not a bad choice. $$
Older M3 - If you go this way I would take the E36 model. Most powerful car on the list; can be an excellent track car. $$
280Z - I think I would start with a better platform. Parts and community support are not very good for this vehicle.
I'm with you on being safe especially when we're talking about our kids. First time I took mine to the track one was in his Mustang Bullitt DD and the other in a track rental Miata. Guess who I was worried more about in the fast turns.