Hi Amy,
This is a very complicated issue as, by the tone of your letter, I am sure you are aware.
Of course, you are concerned with your daughter's health. That is reasonable. It is also reasonable to be concerned with what you define as "superficial" issues of obesity. Teasing and taunting by peers is very traumatic for children. Naturally, you want to spare your daughter any trauma. Be careful, however, not to impose your own predjudices and anxieties on your daughter.
I remember when my son was a freshman in high school. We moved when I got remarried and we had a baby on the way. I was very concerned regarding his adjustment because I had also been moved after my freshman year in high school due to my parents' divorce. I was very traumatized by this move and never really readjusted. I was very anxious for my son. As it turned out my son reacted quite differently than I had and the move turned out to be a great change for him. He is currently in grad school and maintains close friendships with peers he met after our move in his sophomore year of high school. My son was active in the band and this affiliation provided the opportunity for him so form many close friendships. Bottom line: My son and I were very different people and my experiences were not the same as his.
As far as the eating. You are in charge of the food. Be in charge. If your daughter is sneaking food without your permission, than discipline is required. Traditionally, age 6 is not an age of rapid growth. Additionally, doctors follow trends. When your daughter's weight goes up to the 80th percentile in weight but is unchanged in height, this is a red flag.
It is certainly possible that your daughter's height will catch up, but you are correct to be concerned. As far as your mother in law's height: The fact that your daughter has alway's been in the 30th percentile for height doesn't mean she will not grow considerably during puberty. The odds are your daughter's height will be some average of your husband's and yourself allowing for differences of sex. For example, is you are 5'2" and your husband is 6'2" your daughter will probably be about 5'6"-5'7" at full height. Most tall people were always tall on the growth charts. Most petite people were always petite on the growth charts.
Being overweight is unhealthy. Obesity leads to diabetes, coronary artery disease and many other ailments. Obese children are less active and therefore are not building strong bones for the future. Building a strong skeleton requires weight bearing activities and some high impact activity.
There is a general trend toward obesity due to over processed, calorie dense foods that are readily available to us in fast food establishments. With frequently both parents working, we grab these "quick" meals which are calorie and fat laden and provide little nutrition. Also when your daughter eats these foods, she gets a quick increase in blood sugar followed by a crash resulting in increased hunger and the cycle repeats.
I have heard people say that when you are shopping, shop the periphery of the store. This is where most of the "whole" foods are. Also provide protein with each meal. A breakfast of shredded wheat with 1/2 banana and 1/2 bagel with cream cheese with a glass of o.j. will provide hunger satisifaction and a slower drop in blood sugar then will pop tarts or a donut. Smoothies are another great breakfast choice. Even a slice of whole wheat pizza is a good choice. You can make your own pizzas whith whole wheat bagels. Snacks should also include protein. For example, apple slices with peanut butter. I realize most schools don't allow nuts which is unfortunate, however, you could send celery sticks with cream cheese or even small slices of cooked chicken breast with grapes. Soups are a great lunch and at dinner provide lean meats with vegetables and a small portion of starch. (potato or rice) Popcorn is a good snack option. Just be careful of the microwaved versions which have a lot of salt and fats. I pop corn in a pan on the stove with just a little cannola oil. I add a very little butter and salt and toss. It is delicious, filling, provides needed fiber and you can snack for a long time without consuming too many calories. Also be very aware of the calories your daughter drinks. Milk should be low fat and limit juices. Particularly the ones with added sugar. (the "cocktails") Obviously soda should be a very rare treat. You would be shocked how many calories some people drink. Just make sure your daughter well hydrated with water or low calore/high nutrition drinks if she is very active or it is hot. (Gatorade is loaded with calories)
Another problem may be that your daughter is bored. Today, so many people are concerned with safety (child abduction, accidents etc.) that children are not allowed out to play in the neighborhood as they used to. This is unfortunate because many social skills were also acquired this way. Make sure your daughter has enough activities to keep her occupied.
Best wishes with this issue which concerns many of us parents today.
J.