You really don't need any extra money at all, seeing as your kids each have a $50 gift card. We never spend that much money when we go to WDW, especially when on the DDP. DD18 and I are very thrifty! We are pretty hearty eaters, but DD and I are happy to split a breakfast platter each morning (or one of the pancake platters) so we only use one QS credit for breakfast, and split our lunch, too, which means only 1 QS credit for lunch. That leaves us each a TS credit for supper, where we each have our own meal. The snack credit gets used for a mid-day (or late night) Mickey bar or something. Not knowing how much your sons eat, you might want to bring breakfast food along with you, but if you pack cereal and plastic bowls, you can buy milk (quarts) at the hotel and have a cheap b'fast in your room. That'd give everyone his own QS credit for lunch, TS credit for supper. Don't forget the snack!!
We never spend very much on souvenirs. Honestly, it's all so overpriced and hard to pack in suitcases that we just don't spend a lot. Occasionally DD will do pick-a-pearl (which was about $16), once she bought a dress for $25 (she was sopping wet and see-through as she got off KRR) and has worn it dozens of times here at home. Once we bought a small stuffed Minnie Mouse that is currently packed to return to college on Monday! I have a $3 cutting board from POP Century. Other than that, our main "expenditures" have been on pressed pennies (51 cents each) and compressed washcloths (about $4 each, not sure they do them anymore). As you can see, with your kids having $50 each, they have enough to cover a decent souvenir AND extra snacks if they so desire!
I always look at it this way: If you can't say no, you're gonna spend a bundle. If you keep an eye on the budget and don't want to go home to credit card bills, you'll be fine!
( The only thing you didn't mention was money to tip the housekeeper, but honestly, that's not required. Housekeepers are paid at least minimum wage and Disney doesn't consider it a tipped position, so tipping is at your discretion. Can't make suggestions as there's a tipping thread dedicated to this... we don't talk about religion, politics, or tipping in "polite" threads on the DIS!)