My kids have always been much more willing to try new things that they had a hand in preparing. We have a garden every year,and always try to plant a few new things. They are more willing to try them if they planted the seed, watered it, harvested it, etc.
There are some great kids cookbooks out there, some even geared to younger kids like yours. Maybe you could check out some story books about food and after reading them try the foods mentioned in the book together. It's sneaking new foods in under the guise of play.
Younger kids always love to help in the kitchen. Kids as young as yours can help tear up lettuce for salad, mix things with supervision, etc. After helping prepare it, new foods might not seem so scary.
Good luck with your kiddo!