I know what you mean but the carrots don't have to be so obvious. If DD is working hard but starting to look restless I might offer her a treat like a bowl of strawberries or, yes, M&Ms (lucky she loves fruit over chocolate but she is also a sugar hound). Interestingly she took on board a seminar at school on working for 40 mins and then taking a break to do something enjoyable and that in itself is a treat. We don't reward success as such but try to get DD to delay doing stuff she loves to do in her break, be that Minecraft, watch some anime, listen to music, skype with friends...It's actually what most adults do.....get the chores done then relax and do something nice. That's if they can find an end to the chores.