I'm personally a fan of hybrids for beginners. Wider tires seem to inspire confidence and permit light off-road use, while remaining slick enough that pedaling down the street isn't a chore. They're sturdy enough to haul things (like kids) without being overly heavy. And, they can be purchased for a reasonable price.

To second what others said, it might be best to collect some different options and let your wife try them out. Depending on where you live, there might be some good used options already out there. If you'd prefer to buy new, getting a slightly better bike also means that you can recover some of your money by selling it if it ends up collecting dust.