Hi C.,
We recently went to the feeding clinic at Childrens as my son had barely gained 1 1/2 lbs in 10 months. At age two he was diagnosed with a kidney disease -- RTA. NOT that I think your child has this but most of the recommendations we got from Children's was the following:
Make every meal and snack a family event at the table. While the child does not have to eat the parent must (to model eating). Also do not allow grazing between meals and snacks. It was also recommended we play games related to eating.. making food in a play kitchen (you can make one out of a large carboard box -- or even draw a picture of a burner and use pots and pans), play restaurant.
The other suggestions was to add fat to foods... whole yogurt (yo baby, brown cow etc), butter, cheese, sour cream, continue giving her whole milk. My child likes oatmeal (not the instant) I make the minute oats with milk. Basically, increasing it's calories. I add butter, brown sugar, milk and coconut flakes from Vitamin cottage (not sweetened like at King Soopers) -- another high fat food). We are also adding instant breakfast to his milk, HOWEVER I am not sure I'd recommend that at this point... since I am not a doctor and you want to continue to encouraging your child to get most of her nutrients from her food.
Are you grinding up her food at all? Is it a texture thing? What does she like? I am trying to remember but it seems like my son still liked garden burgers at this age. He also really likes taquitos (with cheese and quacomole). My friends 15 months old likes these too. Does she like Pasta? My son loves spaghetti and pretty much all pasta. If she likes red sauce, add some cream cheese to make it creamy (read add calories). You can also high veges in red sauce (steam some spinach/carrots and puree them and add to sauce). Add rice to scrambled eggs. My son loves grilled cheese (new thing) and dipping in sauce (spaghetti). Continue to offer veges, try preparing in different ways... eventually (I am hoping) they will eat it.
Another resource is the book "First Meals" it has great ideas all the way to age 5. I would also recommend a kids multivitamin.. either liquid drops or something she will eat. I found that my son would eat the gummy ones (I just gave him one, but you might want to check with your pediatrican). Although someone recommended a powder, that might be a good option as well.
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L.