Shatanand was the son of Sage Gautam and Ahalya and not Indra. In Balakand Sage 51 Verse 1 and 2, he was told to be the 'eldest' son of Sage Gautam and Ahalya.
Sarga 51 Verse 1 and 2
तस्य तत् वचनम् श्रुत्वा विश्वामित्रस्य धीमतः |
हृष्ट रोमा महातेजाः शताअनन्दो महातपाः || १-५१-१
गौतमस्य सुतो ज्येष्ठः तपसा द्योतित प्रभः |
राम संदर्शनात् एव परम् विस्मयम् आगतः || १-५१-२
On hearing that sentence of that intellectual sage Vishvamitra, the highly refulgent and the great ascetic Shataananda is overjoyed, and Sage Shataananda, the eldest son of Sage Gautama, and whose radiance is brightened by his own ascesis is highly amazed just on seeing Rama. [1-51-1, 2]
बुद्धिमान विश्वामित्र जी के वचन सुनकर, महातेजस्वी एवं महातपस्वी शतानन्द जी के रोंगटे खड़े हो गये || १-५१-१ शतानन्द जी महर्षि गौतम के ज्येष्ठ पुत्र थे और तपः प्रभाव से प्रकाशमान हो रहे थे। वे श्री रामचन्द्र जी के दर्शन कर, बड़े विस्मित हुए || १-५१-२
These verses are just after the incident where Lord Rama freed Ahalya from her curse and then Sage Gautam and Ahalya unite and then Lord Ram along with Lakshman and Vishwamitra goto Mithila where they meet Shatanand. So, by this time Shatanand was already there, and he is being described as eldest here only, meaning that there were other son/daughter of Gautam and Ahalya too at the same time. After the Indra-Ahalya incident, Sage Gautam has left her so no point any children were born after that incident. Hence, Shatanand was born prior to that instance and is the son of Sage Gautam and Ahalya only.