You can transfer credit from your parent account to a child account by following the below instructions:
Step 1: Login to your parent account
Step 2: Click on BILLING from the top menu
Step 3: Click on TRANSFER CREDIT
Step 4: Select your child/client account from the drop down list and select credit amount and click on TRANSFER
NOTE: You can also transfer credit by clicking on ADD CREDIT from Actions column beside your child account.