Bal Diksha is a rite of initiation for children in Jainism. At this stage, the child is just beginning the spiritual path and is not yet a full renouncer.

• In most traditions, a bal (child) undergoing Diksha wears ordinary clothes, which can include underwear. The purpose at this stage is instruction and purification, not complete ascetic attire.
• The stricter ascetic dress codes come later, when a person takes deeper vows. In Digambara tradition, much later renunciation is associated with naked renunciation, while in Shwetambar tradition, white clothes are worn by monks but the stage names and practices differ.

So yes, underwear can be worn during bal Diksha, since it is an introductory phase and not the final monastic code. If you want, I can share more details about how Bal Diksha is performed in different Jain traditions.