Deductions & credits

The IRS states that "the credit amount has been increased from $2,000 to $3,600 for children under age 6 and to $3,000 for children ages 6 to 17." Am I not understanding that correctly? So I have 3 children whom I received $6k total for on previous tax returns but now this year I am receiving $4,800 up front in payment and will receive $4,800 when I file in 2021. So my concern is I will end up owing because of this and I need to increase my withholding but I don't know how much...