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.
Even greater cause for concern is that a large child of 15 months who is in the 95th percentile for weight can easily approach 30 pounds. At that age a child's body is not developed enough nor likely to be tall enough for this type of seat. A good thing is that of the 28 booster seats with the lower allowable weight limits, at least half also include a minimum age limit of three and others include height limits that are more typical of a three-year-old.