Hmmmm... my brain might be on backwards tonight, but, some boilers won't run the circulator if the boiler water temp is too low. The idea behind that is it allows the boiler to always have heat for domestic hot water. So, I'm thinking when the circulator comes on it draws the heat down, the pump shuts off. In my mind the answer would be to RAISE the low temp, not lower it. Then you'll have more BTUs sitting in the boiler waiting to go. Also, depending on how the pipes are plumbed, you might partially close all the other valves and leave the one in your daughters room full open. That will drive more heat out there. If they are all in series, that won't work.
At one point you say "by that time the thermostat is satisfied". It's possible that there is simply too much radiation on that zone and the only real cure is to run new pipes to your daughters room.