A child restraint system for use in vehicles comprises a seat portion and a base. The base secures the child restraint system to a vehicle seat with a vehicle restraint such as a seat belt or a LATCH system. The base both engages the vehicle restraint and includes a force-multiplying mechanism for tensioning the vehicle restraint. Tensioning the vehicle restraint secures the base by forcing the base against the vehicle seat. An embodiment of the base includes a platform configured to rest on the vehicle seat, an upright member joined to the platform, and a tensioning device, including a force-multiplying mechanism, for tensioning the vehicle restraint.