There's no way they wanna send baby Hiyuki there, they gotta be trying to find some way to replicate the flame bone or something, because there is NO. WAY. they're trying to send in a diaper baby lmao
The weapon may be autonomous to a degree or can be wielded by another clan member if the baby is there. The baby would be more of a key rather than an active wielder. Maybe. I know that's not how it was used in the present but I don't see much else that would make sense.
Neat few bits of lore revealed. Datenseki seems to be an iron mineral? And Flame bone isn't a datenseki-based weapon apparently. Why aren't they looking into whatever process created it to try and replicate it? Although they call it a "mystical weapon", so it may well be datenseki without them knowing it. IIRC it was introduced way back as a "predecessor" to the enchanted swords, which kinda implies it's of the same origin.
Also why don't they just run both plans in parallel, not like they have a better use for the datenseki anyway.