you know, those percentage charts are HIGHLY unfair to smaller children. it is exceedingly unfair to compare children to other children their age because each child is different!
i would ask you these questions:
is she healthy?
is she happy?
does she eat well?
doe she eat 3 meals plus snacks?
does she drink plenty of water?
does she have a bowel movement at least once a day (or every other day at most)?
does she have wet diapers (unless trained)?
does she have good color in her skin?
are her eyes sparkly and full of life?
is she growing at her own rate, gaining weight, even if its mere ounces?
those things will probably tell you a lot more about her state of health and weight than any doctor could. to say that her weight has fallen to the 10th percentile only means that compared to other kids her age, she is lighter. so what? did she gain weight or remain at the same weight during that period, or did she lose weight? if shes losing weight, obviously, do something. add a higher fat milk, cheese, peanut butter/nuts, those types of things have higher fat content, and especially with nuts, healthy fats. offer her bigger servings. maybe she would eat more if it were offered.
however, if her weight has remained the same, or gained, even if its just an ounce or two, i would just remain unconcerned. who cares if she doesnt 'measure up' compared to her peers. shes healthy for HER, not for someone else. you will screw up her balance and her hunger cues if you make her eat when shes not hungry, or make her finish her plate when she is full. you know ?
i have a friend who is petite, her husband is TALL, but skinny as a rail. ALL THREE of her children were just like yours; always under weight compared to their peers. they are perfectly healthy, and even eat junk food now and then. there is nothing wrong with her kids. they are just who they are with the body structure that they were made to have.
those charts are just so unfair. i would pay more attention to the actual number of her weight, and height, and making sure that she is gaining or at least staying the same over time. as long as shes looking good, acting good, and energetic, shes fine. if shes starting to act wierd, her body looks sick, then start worrying. especially when you and your whole family are making lifestyle changes to be healthier, i wouldnt mess with a good thing. remember that any addition in calories should be with fruits, veggies and beans/legumes. those will keep her healthy, while adding anything she may need in her diet. dont be tempted to add junk food :P but you probably already knew that.
by the way, those beans also would have higher calories and yet healthy ones!! :) split peas are a great soup base. add veggies and meat to it, and presto, you have a GREAT stew! :)