We are a family of 4, relatively small eaters. I always figure about $100/day. Our girls are 5 and 3. The way I figure it is approximately $30-35 for CS meals - so if you do 3 CS per day you get $100. If you throw a TS in that can run around $70 (including tip), plus 1 CS meal for us and breakfast in the room, you're still at around $100 for that day.
The things that add up for us tend to be breakfast because my husband will get the a la carte stuff that adds up--bowl of cereal, bottle of OJ, fresh fruit, jug of milk, a coffee. We usually order two adult CS lunches and share those with the kids. This next trip we are doing the dining plan and paying for it because it will only cost us $700 for our 7 night stay and we want to try more TS meals this time. I added up all the stuff we wanted to try and it came out to around $1100 OOP so it made sense to do the dining plan this time around--plus it will be prepaid and we can relax about it. Normally I start to wince each time my husband gets back in line for another piece of fruit--now he can use a snack credit!