My 18mth DD barely seems to eat anything! POrridge for breakfast, maybe a nibble on something small mid morningish, lucnh today was a mini pork pie (seriusly, that was it), tiny glass of milk midafetrnoon, not much dinner (mostly corn on cob, bit of rice, bit of meat).
She drinks water with breakfast, midmorning, lunch, afternoon, dinner and bedtime. Not fussed by milk, especially if she's eating yoghurt or cheese at some point in the day.
She is growing taller and taller and seems to be mentally and physically developing, so I guess she's eating all she needs. I wish she'd eat more fruit, but that's my fault for not offering as I don't eat it. We love veg though!
HTH, it does seem that lots of them don't eat much!