Rule to issue commands based on upper and lower temps

Use

when
    Item kidsroomOccupancy changed Or
    kidsroomTemperature changed
then

to trigger your rule. That way the rule gets triggered whenever the temperature or the occupancy changes and you can calculate the proper state of the heater.

Incorrect. Rule triggers are event based, not state based. The rule runs when certain events occur, not while Items are in a certain state. The trigger you are attempting to use is invalid

That being said, you can do a simple check like the following:

if(kidsroomTemperature.state > 77 && kidsroomHeater.state != ON) {
    kidsroomHeater.sendCommand(ON)
}
...

Note how I’m using an Item above instead of executeCommandLine. You should have a Switch Item to represent the state of your heater/airconditioner and when this Item is set to ON make the call to executeCommandLine.

See:

Again see the Proxy Design pattern. Essentially you want to represent the state of the air conditioner using a SwitchItem. Then have rules to interact with the other Items and translate commands to the Proxy Item into commands to the device (i.e. your calls to executeCommandLine) and translates changes to KidsAC into updates to the proxy item.