Is this possible through TurboTax...take the full child credit on their 2021 return (3k) instead of monthly payments starting in July?

No, TurboTax is not involved in any way with how the IRS will be disbursing the advance child credits for tax year 2021.

what would be the difference? I am curious. 

the payments would be an advance against the credit, so instead of getting a $3000 credit, increasing the refund by that amount next year, you would get $1500 across 6 months with the remaining $1500 left to increase your refund.  Isn't money in our pocket better than money in the IRS' pocket?

ps - in the last few days IRS is saying they may not be able to begin this in July in any event - too many tax law changes to deal with.  The law technically provides for 'periodic payments' and not 'monthly payments' so 'periodic' could be one time 🙂 

Not through TurboTax, but once the IRS website is set up you WILL have the option to "opt out" of the monthly payments.  If you do that, you can claim the full credit on your tax return when you file your 2021 tax return (about a year from now).