Can I use my JumpStart account to fund my trading account on SCB?

Yes, you absolutely can; but there are better brokers out there for your information such as IBKR.

When you apply for the online trading account, there will be securities settlement accounts that will be opened as well, for the currencies that you selected in the application form. You need to transfer money into the settlement accounts before you can start trading.

For the SGD securities settlement account, you can do an internal transfer between your Jumpstart account and the settlement account.

For other foreign currency settlement accounts, you can either transfer to your SGD settlement account first and do a currency exchange with the LiveFX feature later or do a transfer between your jumpstart account and the settlement account at the bank exchange rate at that time.