Been remembering a children's/YA Sci-fi novel I read years back and wanted to identify.

This lady inventor built a time machine that was described in the book as a flying doughnut and I think it opened with her and her two kids viewing some dinosaurs.

One of the kids, a boy, told a friend about the time machine who promptly sold the story to a reporter in exchange for 50 "ecu" which was the currency in the maybe near future setting and then the inventor's home is swarmed by the media.

The climax of the story was a pack of neo-Nazis showing up and demanding the inventor bring Hitler to the present, though like any good "patriot" they don't speak any language other than English.

After dispensing with the neo-Nazis by some means I don't recall the inventor destroys the machine by sending it back in time and dumping it in Mt Vesuvius, the book ends with the family speculating that they may have inadvertently caused the eruption that destroyed Pompeii.