Sounds like my son. I don't recall exact weight/ages but even now at 8 1/2 yo in 3rd grade, my son is the lightest skinniest kid (he's probably hm I think 35 lbs?) I just had to make peace with the fact that he was going to be a little kid (my husband and I both were through grade school so why do we think we'd have a big tall heavy kid?!) and, this is important:
As long as she is eating a healthy selection of foods and growing normally (developmentally) and active, engaged, able to do things, and her weight (even if slowly) increases, and there is no *unexplained* weight loss or lethargy, then she's fine. Repeat: she's fine!
She'll just be a little thing which is a refreshing sight with a huge majority of kids being overweight. Be thankful too when you need to pick her up, that she's 20, not 40 lbs!!
Oh yeah,the pedi told me to put butter on his foods when he was a baby/toddler. That backfired on me because if I touched his bread or toast or pancake with butter, he would not eat it!!! I figured a half slice of bread eaten without butter was better than one bite with butter eaten. Ever so slightly I would take things to the next level (little bit of melted butter onto hot pancakes for example) and now he eats butter fine on anything. He could do without, but doesn't fuss or reject if I add butter.
Also, he would NOT eat peanut butter when he was a toddler! When he went to Kindergarten and had the last lunch shift (they had snacktime mid morning) I didnt feel comfortable sending cold cuts sandwiches, and he wouldnt eat peanut butter, so I got Nutella (oh that stuff is yummy! but it was for him, not me, I certainly dont need it now!) and he willingly ate it. So as the year went on, I gradually mixed more and more peanut butter into it. By 1st grade he was fine with PB sandwiches. (Now as a 3rd grader who wants to sit at the peanut-free lunch table with his friend, argh! I'm having a hard time getting nourishing, filling, nutrient-dense foods into him again!)
One more thing - foods are so much more appealing when eaten with dipping sauces - ie ranch for veggies, syrup for french toast/pancake strips, yogurt cream for fruits, etc.