After dīkṣā, the child is no longer treated as a “householder child” but as a bal-muni / bal-sādhvī (young monk/nun). So their dress follows monastic rules, not normal children’s clothing.

In simple terms:

• Śvetāmbara tradition

- Monks and nuns (of any age) wear only the prescribed simple white cloth (few specific pieces). - Modern “underwear” as a separate, personal, stitched garment is not kept; whatever cloth they wear is part of their sadhu-vastra (ascetic clothing), decided by their ācārya and maryādā.

• Digambara tradition

- Fully initiated male munis ultimately practice total nudity (no clothes at all). - Where some preliminary stages or female ascetics use cloth, it is minimum prescribed cloth only, not normal householder-type underwear.

• For children specifically

- Once they take dīkṣā, they must follow the same clothing discipline as the order in which they are initiated, under the direct guidance of their guru and saṅgh. - Small practical adjustments (health, local law, etc.) are handled by the ācārya, but the principle is clear: they give up worldly dress, including normal underwear, and adopt ascetic dress (or nudity in Digambar male munis) according to their tradition.

If this is about a specific planned dīkṣā, the exact details (which pieces of cloth, from which day, what transition) should always be confirmed directly with the initiating ācārya or guru, because each saṅgh has its own strict, written maryādā.