We budget $20 a meal when on
DDP. That covers the tip and if I get charged for the coffee I almost always add on. Some meals are more some less, but $20 seems to be a good average.
If you drink adult beverages they add up fast.
About the quarters. Get a tube of m&m minis from any candy aisle. Save the tube. It is the perfect size for quarters, and a great way to carry them in the parks. Pressed pennies are fun, and were often a life saver for us when the kids felt they needed something, anything, just felt the had to spend money to make the experience complete.
I think we spend under $100 on stuff on our last trip, but my kids go often and they don't change out the merchandise too often. If this is a first trip, or a once in a long time trip I'd budget $150 per child if they are in the 5-12 age range. For adults it really depends, I tend to be happy with one tee shirt (under $50). Young teens have fewer options, my daughter didn't get anything on this last trip (although that is in part because she already has so much, it was also because there was very little aimed at her age).
We always try to direct our kids towards one bigger item instead of a million little things. Girl's are always easier, for a girl under 10 there are really nice dolls sort of like the American Girl dolls. Dresses, purses, charm bracelets, and a million types of jewelry in every price range. Also the build your own remote control car thing seems really fun, and my daughter would have loved it a few years ago. That can easily go into the $75-$100 range but is something that would get played with outside of WDW too.
That said two 'little things' every kid needs are Ears and something that lights up for the parades/night time.
Oh and did you budget other tips yet? Valet, Bell services, Mousekeeping, and Cab fare are all items I budget for.