Does minor need a bank account before opening a demat or trading account?

No, a separate bank account on minor's name is not required.

Simply add minor to any of your (Guardian) bank account. This is sufficient to open demat / trading account.

Please do not fall in the trap of bank officers who will try hard to open a separate account on kid's name and later charge maintenance fees.

Anyway, for tax purpose, minor income is added to the parent's income, it doesn't make sense to have a separate bank account.