The biggest Herbie presence in the parks is at
Disney's Hollywood Studios. Herbie makes an appearance in the
Lights, Motors, Action! Extreme Stunt Show. And the last time I was at the Studios (in December) there was a life-sized Herbie in front of that attraction that children could be photographed with. If you decide to visit the Stunt Show with your little ones, be aware that while the show is not particularly scary, it is LOUD. If your kids are noise sensitive, you might want to wait until your next trip.
As for the merchandise, the selection in the shops at WDW is constantly changing. There will very likely be Herbie gifts available at the shop just outside the Stunt Show, but don't promise anything to your kids until you actually get there. To find out about the store at
All Star Movies, I would call the resort directly at 407-939-7000 and ask them to connect you to the gift shop. You can also get WDW park merchandise sent directly to your home by calling the Walt Disney Mail Order Merchandise Department at 407-363-6200.