Dear A.,
First off, I would like to say you are doing some *great* things as a mom. You are breastfeeding, co-sleeping, baby wearing, and many other things that say to the world that you love your kids and being a mom. So please pat yourself on the back right now!
If your child has always been small, but growing taller and reaching developmental milestones...then she is HEALTHY! I have experience with this issue. My daughter was born at a healthy weight but then leveled off off at twelve months and fell of the weight charts. I was breastfeeding and not giving solids because I was unsure how to start. My pediatrician created a lot of unnessecary drama over this and I won't go into all the details but...my daughter is a petite little girl still at age 4! She eats a ton of healthy food and she now weighs just 30 pounds. I'm 5'3" and my husband is 5'6" and we both have small frames...something I kept telling my ex-pediatrician who only wanted to focus on my daughter's weight.
Please don't take the WIC office advice on nutrition!! Carnation instant breakfast milkshakes are a poor choice for nutrition. Read the label on that drink. It is full of partially hydrodgenated oil, sugar, and synthetic vitamins.
A better choice is to offer high fat, densely nutritious foods like: avacados, pitted olives, nuts or nut butters. If you want to make a healthy shake for her, blend some organic soymilk (Publix Greenwise Brand, plain), a frozen banana, and some peanut butter. Add a little honey for sweetener and blend in a blender or hand mixer.
As long as your are feeding your family whole foods, you are on the right track. Always read the label and look for ingredients that say there is *real* food in it.
Hope that helps :)