I might be able to be of some help since I've been raising a good chunk of our birds inside, and also hail from Indiana. I raised each group with a heating lamp until they were about 4 weeks old before taking them completely off the heat. At about 1 1/2-2 weeks, I moved the lamp away about twice as high so they don't overheat. If your duckies start panting, they need less heat or more of a covered space to cool off. I don't have any experience with heating pads so I can't speak about those, but would also like to know more on that issue.
When you start taking your ducks completely off of heat, monitor how they act. If they start huddling together and lying around more often, then they are cold and need more heat. As far as moving them outside goes, I've been aiming around 7-9 weeks old for our current weather conditions. My first set went outside about a month ago and they were exactly 7 weeks old but had their feathers in and had an excellent reaction. I'm currently waiting any moment to move my second set outside. They're almost 8 weeks old but they're feather development is behind the first sets.
Paying attention to their temperament is going to be the best way to read your birds. If they start panting, find a way to lower the heat for them. If they start huddling together all the time, especially on top of each other, raise their heat! Always make sure they have water and never leave food out when there is absolutely no water around. Keep up on the cleaning, don't let it get to a point where you can start to smell ammonia. Hope this little bit helps out! Good luck and most importantly, have fun!
Sounds like they are too warm what breed and what temp in your brooder?