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.
Many booster seats wouldn't be appropriate for a smaller toddler because of the minimum height and weight. While some high-back booster seats have a minimum weight of 30 pounds, there are lots of other booster seats that require a child to weigh at least 40 pounds before use. From a practical standpoint, parents tend to have an easier time keeping their child in the harnessed car seat altogether.