That lamp is probably way too hot for that small box.
I'd put a 60 to 100 watt incandescent bulb in there, leave the wire guard off and set the lamp right on the wire mesh on one end of the box.
Put a thermometer on the floor of the brooder under the light and see what the temp is, then put the thermometer at the other end of the brooder and see what the temp is there.
The temp should be about 90 degrees right under the light and 70-80 degrees at the cool end of the box.
Use a different wattage of incandescent light until you get the temp right...or a dimmer extension cord to adjust the heat output.
They need to be able to get away from the heat if they are too warm or they'll get sick.
Keep the food and water at the cool end of the box.
You're going to need a bigger box, or add onto that one, really soon.