Meadow, I tend to pop up at the sight of a picky eater's thread and so I'll add my tuppence worth here too! Though everyone's given you such good advice already. I have had a picky toddler (as a baby she was no problem) for about a year (she's 26 months now) I have tried all sorts and tried hard to listen to the advice to the tune of 'she'll eat when she's hungry' but I do know how hard it is to think like that all the time...
Some useful tips I've picked up along the way are:
Snacks are GOOD, they refuel the child and keep their blood sugar levels up, so don't try and force a 3 meals a day routine when they are this little - a bit of a nightmare I know but its a better and more realistic way for them to eat...
A child's stomach is about the size of their clenched fist - thats not v big so portion sizes can be v little and still satisfy their hunger without you realising it (this is just a good one to keep in your head when your rationality is sliding!)
While milk on its own is seen as a bit of a 'filler' without containing all the various nutrients other than fat and protein, something like a fruit based smoothie is better. I use strawberry bio yoghurt (no additives) mixed with milk to which I add her vitamin drops (yes I feel like they help too) and her echinacea drops (my magic potion!). I feel that this with a slice or two of cucumber (with veg its either cucumber or frozen peas at the moment - and I MEAN frozen- she won't touch them cooked!) and some bread or rice cakes (or, funnily enough, Ryvita is a fave!)makes a pretty balanced meal if you break down the nutrition. So, basically, you can find what you may consider to be a 'weird' meal for them that they will actually like! Was interested in AliH's view on iron deficiency though - had never thought about it affecting appetite...
Also, I have heard that its better to view a child's diet based on a week's worth of eating rather than a day - the reason being that one day a child might just eat a lot of protein, but another day just carbs. Its better to look at the bigger picture to work out if they are really getting everything they need...
I have been duped into cutting food into smiley faces and trying the junk food is better than no food theory but if you can just take the panic out of the situation it becomes much easier for everyone (I'm saying this as much to myself as to you!!) If you really think there's a problem then talk to your HV - but I think that dd was 21lbs on her 21 month check and they weren't slightly worried though I voiced my concerns to them... She's still a bit of a string bean, v tall and gangly so I'm now hoping she'll become a supermodel and never have to worry about shoe-horning her big bum into a pair of jeans like her Mum! But joking aside I know what you're going through and hope everything turns out well for you...