I don't think it's necessary to bring something to cover your own hair, unless they are orthodox (in our synagogue, women only wear hats on the High Holy Days, not Passover), but it is respectful for your son and husband to wear a yarmulke since all Jewish men do, no matter what division of the faith. If a non-Jewish man enters a synagogue, it is considered a sign of respect for him to put on a yarmulke, the same rule exists for any religious function. Ultimately, if they want them to wear it, they will provide it (since non-Jews usually don't have one floating around), just let them know they may have to. Flowers would certainly be appropriate if she (understandably) doesn't want you to bring food. The rules of kosher are heightened to the tenth power during Passover, so she wants to make sure that you don't accidentally bring anything that breaks kashrut since Jews have to clean their entire home right before the holiday to get rid of anything that isn't kosher for Passover.
Just have fun with it and take cues from your hosts, as with any function you'd be invited to. The seder may or may not be very long, it depends on the family. Since they are religious, I'd say to expect a long seder before the meal, and the seder will probably continue after the meal as well. My family has always had non-Jews at our seder because my aunt married a Catholic man. We have had his family at many of the meals, and when she was in college she would bring friends with her too. They always enjoyed it because it was a new experience. Don't expect anything fantastic food-wise, especially with dessert (although non-Jews seem to end up loving matzah, a fact Jews can never understand because we dread having to eat the stuff for a week). Passover food is infamous for being horrible, even if they go to a kosher bakery for the desserts they tend to be dry and relatively tasteless, it's part of the holiday
Here's a link to all you might want to know about Passover:
http://www.jewfaq.org/holidaya.htm