What are the tax consequences for both myself and my child if I loan the child $ 800,000.00 to start a business for himself.
You can make a gift without paying gift tax although it needs to be reported, and your child can make gifts back.
If you treat it as a loan, the IRS expects you to conduct your business affairs in a businesslike manner, which means charging interest. If you don't charge interest, or charge less than the minimum rate, the IRS will require to pay tax on the interest income you would have received if you charged at least the minimum rate ("imputed interest"). See this video.
If you do charge interest, your child can deduct the interest as a business expense.