No real rigidity here at all. DD (almost 6) has a snack at school at around 10am, lunch around 12, snack after school (around 4), dinner 5:30/6 and usually a bedtime snack (often cereal or fruit, occasionally a cookie or other treat). She has water or milk in the morning, juice with snack and lunch (diluted) and more diluted juice or milk with dinner. She will always ask for food or drink when hungry/thirsty.
Now DS (just turned two) is much pickier. He almost always drinks enough (no milk, only water or his special fruit smoothies I make PACKED with nutrition) and has some often throughout the day. We have a relatively set breakfast time and dinner time (at least on weekdays), lunch is hit or miss (often skipped) and he gets an AM snack (only if he asks) and PM snack after his nap. Both of those are more like mini-meals sometimes since he doesn't really do lunch. So like those breakfast cookies I've posted, a bagel, pistachio nuts (his current obsession), yogurt, etc.
He's been refusing both fresh fruit and veggies too, save for the occasional apple slice or banana. So LOTS go into that smoothie. Plus he's good for a pouch of Ella's Kitchen or Plum Baby fruit/veg mix 5/7 days a week. LOTS of nutrition and hydration that way and he thinks it's fun to squirt them into his mouth!