Hi Clare!
You do not have to use all of your Disney Dining Plan meal credits at a particular meal, and you can definitely share! Many guests do that.
Let's say you are staying for four nights. In that case, each member of your party will be credited with four counter-service meals, four full-service (Table Service) meals, and four snacks. All of those meals will be put into an individual "meal account" to be used by the specific individual. Meals in your account can be used whenever you would like. You can do a signature dining experience and use two Table Service credits, or you can skip a Table Service meal one day and do two the next.
You did not say how old your youngest is. Just remember that if he or she is less than 10 then you will eventually be sharing a child meal when you redeem his or her Table Service credit.
Also keep in mind that all of your meal credits will expire at midnight on the day you check out. Unused meal credits are not refundable and you can't save them until your next trip.
You can get more information on the Disney Dining Plan
here. Have a magical vacation!