You may want to purchase a Universal product for more children. We will automatically prorate the additional charges. You will not be charged immediately for the additional children: those charges will be reflected on the next invoice. Again, the plan changes impact the next invoice, which remains on the same billing cycle (as your plan will always have the same billing frequency).
To give you a concrete example of how this works, let's say you signed up on April 1 for $10/month. Halfway through April, you decide to add on more children for another $15/month for a total of $25/month. You'll be charged $10 on April 1, the day you signed up, then $32.50 on May 1, then $25 on Jun 1. Here are the details… You have already paid $10 and used half the current billing for April. When you added the additional children, with a total cost of $25, the unused portion of the previous plan resulted in a credit of $5 (half of $10). The cost of the new plan for the rest of the billing cycle (i.e., the rest of April) will be $12.50 (half of $25). Therefore, the total prorated cost of adding additional children is $7.50 ($12.50 - $5 = $7.50). This amount is added to the next invoice of $25 for a total of $32.50. On June 1 onward, if you do not change plans, you'll be charged $25/month.
The prorated amount is calculated down to the second. This means that we will calculate the differences between the two plans based on the exact time you added/removed children and their relation to the current billing period's start and end times.