If the Walkers deducted $850 each month from their checking account for childcare over the course of a year (12 months), the total amount deducted would be:
\[ 850 \text{ (dollars/month)} \times 12 \text{ (months)} = 10,200 \text{ dollars} \]
Therefore, the effect on their checking account balance would be a decrease of $10,200 for the year due to these childcare payments.