All you need is a phone line! All phone lines have a constant flow of voltage, around 40-70 volts (up to 100 volts when it rings!), but you can't just plug stuff into it and expect it to work.
Children older than 5 and more than 40 pounds must ride in a booster seat until the child is 8 years old or is taller than 4 feet 9 inches. Anyone over the age of 8 must be protected by a safety belt.
If you answer no to any of these questions, your child still needs to use a booster seat (some are designed for kids weighing up to 100 pounds). Of course, your big kid may protest that boosters are for babies, so offer extra encouragement to help him feel better about keeping his booster seat a little longer.