They are both correct, the only difference being the level of formality. तुम्हारा (the masculine possessive form of तुम) is more polite than तेरा (masculine possessive form of तू). तू is super informal and should be avoided by learners. Use आप by default; तुम should be fine with children. If someone's fine with you addressing them as तुम or तू they will probably tell you.