Why not put the impetus on them? Have a family conference and allocate one day of the week to each family member to decide on a reasonable menu for the rest of the family. (For the younger ones, you can guide them with this until they get the hang of it.) You will decide the menu for the rest of the week.
Have some rules in place such as:
each meal has to have protein, carbs and vegetables, nothing too fancy;
when the menu has to be written by and if it isn’t, what the consequences are (you get to choose the price of tardy submissions! Too bad if it is their most hated meal!);
no complaints will be entertained since each person gets to decide on part of the week’s menu.
Remember, you are the manager - you decide the rules and you have final veto!