1. Beans (e.g: Kidney beans), chickpeas, vegetables in general. Also spices are handy to have.
2. Depending on the meal, you could cook it vegetarian, and then your family could add meat, etc afterwards. Or just have two separate pans. Sometimes you could offer to cook a family meal that's suitable as well.
3. Protein is found in all sorts of foods in high amounts, e.g: Nuts. In fact, it's so easy to get 55g of protein from beans and nuts alone. Protein is also found in milks (both dairy and non). In fact according to the American Dietetic Association states that a majority of Americans eat twice the amount of protein they need, which has harmful effects. You can find more information on the exact amounts of protein in a handful of almonds, etc, on the Vegetarian Society's website.
4. Haribo, gummy bears, some yoghurts, etc which contain gelatine. Also some dairy products contain rennet which vegetarians tend to avoid (although you have to research this on a company by company basis). Note that parmesan contains rennet, and so isn't vegetarian. Another ingredient is carmine acid/cochineal (E120), found in some sweets (especially red ones).