Reason for white discharge
7year old gets white discharge. I'm worried

If it's like cottage cheese, it's a yeast infection from pH that is out of balance. Monistat is available at a drugstore. Search 'vaginal yeast infections' to see how to prevent them too. I'm assuming this is a girl. Cotton underwear, keep clean and dry, get some panty liners, drink lots of fluids preferably ones like real cranberry juice.
If it's more clear and like stringy glue, answer back.

If there is no odor and only white don't worry. Her internal vaginal balance has been disturbed somehow. This is a natural fluid to correct the imbalance.