My kids room is cold
More often than not, the problem you describe is the result of the problem room having excessive heat loss, not a deficiency in the HVAC system. For example, if the cold room was the only room over a poorly insulated garage, that would explain everything.
It would be useful to measure ceiling, wall, and floor surface temperatures and compare to other rooms.
Some power companies offer a free Energy Audit service that can make these measurements for you, down to every square inch of your house showing you where the thermal leaks are.
Let us know what you find...