In which text the phalashruti for parayana of Balakanda of Valmiki Ramayan has been mentioned? And what is it?
One Phalaśruti of Śrimad Vālmīki Rāmāyaṇa Bālakāṇḍa (aka Ādikāṇḍa) can be found in Chapter 26 of the Bṛhaddharmapurāṇa Pūrvabhāga:
अनावृष्टिर्महापीडाग्रहपीडाप्रपीडिताः। आदिकाण्डं पठेयुर्ये ते मुच्यन्ते ततो भयात्॥९॥
In case of drought, being harassed by serious illness and afflicted by planets, those who read the Ādikāṇḍa are freed from such fear.-verse 9
Translation by me. One thing to note that the Parayana of individual Kandas is not given in the Ramayana itself. In case of Sri Ramcharitmanas it’s given that those who hear and sing the the story of the marriage of Shri Sita Rama, with their prasada, such people shall gain happiness.