I always take the heat off the chicks before moving to main coop. These are my steps...
Hatch
1) baby brooder (95-100F new hatched babies gain mobility)
2) warm brooder inside *reduce heat each week (standard brooder reduce 5F per week)
3) outdoor brooder or "grow out pen" *reduce heat until they do not need any heat (reduce 5-10F per week)
I always get the chicks able to go out with no heat before I ever move them to main coop. I find that the outside brooder is great in helping me get them acclimated to the outside temps. just reduce heat until you reach outside warm day temps, then start turning off heat in the middle of the day for a hr or 2... then stretch that off time more and more until they are only heated at night. then stretch the off time again into the night until you just turn it off for entire day/night (first couple nights I check on them every few hrs or if you have a web cam that works too to check and make sure they don't get too cold)
4) place chicks in main coop at night. be sure that you will be able to visit at first light to see if there is any trouble.
5) watch young ones in the main coop for a couple weeks again being sure that you are there at first light or there is plenty of places for the young one to run/hide if trouble brews.
If I have one that is aggressive towards the young one I pull that one out and place it in a large dog crate for the day.
If many of them are aggressive towards the young one I pull the young one out and place it in a dog kennel inside the coop so that the young one is protected and the others can get used to the addition.
So far this has worked well for me
TIPs for heat in a wooden box with sawdust........
1) Strap down your heat source! do not rely on the "clamp" Tie it down well enough that no matter how you bump, smack, push, pull, etc that heat will not fall or get close enough to the wood.
2) Use a Ceramic socket! DO NOT USE PLASTIC! The plastic ones are not made to be on 24/7 even if it says it can handle a 150W it can not handle 24/7 so spent the extra $2-3 for the ceramic base socket.
3) when reducing the heat do not move the lamp.. reduce the wattage! This helps you reducing the wasted heat, eliminating any need to move the lamp so less chance to fall and start fire and the birds know where the heat is and don't go bouncing around and bumping the lamp and causing trouble.