Very difficult to say a figure because there are so many variables on how your family wants to do their trip.
You will need to pay for transportation to Uni. Using Uber or Lyft figure in that cost to and from Uni plus drivers tips.
Meals... Can add up if you are doing table services for dinners ( including tips for servers), counter services for lunch, resort breakfasts at food court. Also figure in drinks and assorted snacks at parks during the day.
Souvineers... That can add up to be a major expense depending on how crazy you go buying in the gift shops. We always come home with a lot so I budget in extra.
On our trips for the three of us ( and we go usually 2 weeks) I figure in about $100 a day for snacks, drinks, souvineers. Some days we don't spend it all. I usually come home with more money unspent.
All your meals can be planned out by going on Dis sites that list menus and costs. So you can easily decide pretty much what food ahead of time you may be ordering and what that entire costs may be.
Whatever budget Ive saved for my trip I always add about 15% extra for emergencies or unplanned spending.
As far as saving money there are multiple sites online with tips on how to go to WDW and save costs. Bring snacks or travel off site to grocery stores and buy things less costly to munch on and avoid all or most Disney snacks.
Dine at counter service spots rather than table service restaurants. Drink water instead of buying sodas. We bring packets of drink additives like Mio or Crystal lite, etc. to flavor the Disney water which can be odd tasting.
Disney trips are a major expense for sure but planning everything ahead of time can give you a better understanding on a budget... then you have to be determined not to go outside of what you have budgeted.
That bottom line can be hard to stick with.
