I'm not sure I understand correctly, but hopefully I can still give you a good answer.
You say your package includes dinner - are you on the Disney Dining Plan (DDP)? If so, your package should include one counter-service, one table-service, and one snack per person per day. If you have a different arrangement and you get only that one meal per day, just change the numbers a bit.
I'm guessing that you won't be eating table-service (sit-down) meals, so I would recommend about $15-$20 per meal (will probably be less than that, but a comfortable cushion is nice)
Souvenirs are harder to budget for, as I'm not sure what the girls would like to purchase.
If I were planning this, I would prepare a handout for the parents with some information to help them make their decision on how much money to bring.
www.allearsnet.com is an excellent resource. Complete menus for every eatery on Disney property are listed there, along with prices, so it's very helpful for budgeting. I would also include a couple of assorted souvenir prices for reference:
refillable resort mugs are about $12 (good value - functional, money saving AND a souvenir!)
pins are usually around $8.95
t-shirts around $20
hair wrapping/beading would be $30+ depending on hair length
None of those prices include tax, and all are approximate and subject to change.
Bottom line - if only one meal per day is covered already, I would be sending my daughter with $250 and hoping (foolishly) for a little change.