FYI - normally the tax credit is $2000 per kid. This year its $3600. $300x6 =$1800 advance payment on the $3600. So the advance payments will only reduce your normal tax credit at year end by $200. Unless, your income changed signicantly from 2020 to 2021 and you no longer qualify, you can stop worrying about how these payments are going to affect how much you owe at tax time.
And yet the math isn't so favorable if you have multiple kids over the age of 6, especially if your AGI in 2021 phases you out of some or all of the expanded credit.
If your (MFJ) AGI was $150k in 2020, you only need $19,001 additional AGI in 2021 to reduce the expanded $3,000 credit back to $2,000.
In that case, if you had your withholdings set to neutral (approx $0 refund or owed), then every penny of the advance tax credit will be owed back when you file your 2021 taxes. That's $1,500 per kid over the age of 6.
It's surprisingly easy to realize an additional $20k or more of AGI in a year through some combination of raises, bonuses, side hustles, and diverting some pre-tax savings to post-tax (e.g. Roth conversions).
If your AGI is identical in 2020 and 2021 (and doesn't exceed $150k), then each $1,500 you get in advance payments will be partially offset by the additional $1,000 from the expanded credit. So you'd owe $500 per kid over the age of six.