There are three basic types of cell phone plans. First is the prepaid plan, where there's no long-term contract, and you simply purchase the number of minutes or messages you want. Generally, such plans cover one phone. The advantage to a prepaid plan is that you control how many minutes your child can talk each month, and how many text messages he or she can receive. You're also protected from unexpected, expensive bills if your kid spends way too much time on the phone.
The other two basic plans are both contract plans; one covers a single line, while the other covers multiple people and phones (aka a family plan). Under these plans, you pay a monthly fee for a set period of time -- typically one or two years -- and receive a certain number of minutes and/or messages each month. If your plan covers one line, that line gets to use all of those minutes and messages. If you purchase a family plan, the minutes and messages are shared among phones. If you exceed your plan's allotted number of minutes and messages, you're billed extra -- although you can also pay to receive unlimited minutes/messages. The main benefits of contract plans are that they often include free phones, and may be cheaper overall in the long run.
The cost for these plans varies greatly, depending on your carrier, the phones you select, the options you choose, the length of your contract, etc. It also makes a difference if, say, your cell phone is bundled in with other household services like cable television and Internet. However, in general, if you have several phones, a family plan is the most cost-effective.
If your child is new to cell phones, or you've got one on the irresponsible side, most family plans allow you control the number of minutes, messages and downloads allowed per line, which means no unwelcome surprises when the bill comes in the mail. Other parental options include blocking certain numbers from specific phones and blocking usage at certain times of the day.