Does she actually have "boobs," or does she mean her nipple area on a standard (i.e., somewhat flat) 7-year-old chest? She could be beginning to develop, though age 7 is a bit early. If it is breast development, she might feel like under her nipples it feels somewhat hard.
There is a lot of estrogenic food in modern diets. It's possible she is coming on early because of what she is eating. That said, though, a friend of mine a generation ago got her first period at age 8, and that had nothing to do with eating a lot of soy, etc. Breast development can come about 6 months before the period, I think.
You could take her to her pediatrician and get a blood test to see what her hormones are doing.
Due to processed foods and hormones in meat our children are developing earlier and earlier these days. Its a painful growth process that all girls endure at some point. I would encourage you to take her to the doctor and get things checked out. It will ease your mind as well as you will KNOW how to proceed. Better safe than sorry is a good motto. Best of luck to you. Keep us updated