For kids that small, I think you should do a good behavior jar.
Each jar starts empty. Each full jar represents a final goal/prize/reward. Doing daily rewards can get to be...well, exhausting.
So, get two large mason jars and enough marbles or small rocks or whatever (all of this can be purchased at walmart). Then, when the child does something good, they can put a marble in the jar. When the child does something bad, they must take a marble out of their own jar. It will help them to see that they are earning a goal for more than just today and also encourage positive behavior. For example, let's say you ask your 5 year old to pick up his toys. If he starts to complain, remind him that doing this kind of chore will put a marble in his good behavior jar. But if he whines, throws a tantrum, or refuses to help, then he must go and take a marble out. I used to watch a little boy whose mother started the good behavior jar around the age of 2. It was devastating for him to have to remove a marble, but an amazing event when he was able to add one or two for doing things the way he was asked...or without being asked.
I think your husband has the right idea, but I think it's almost too thought out. They are little. Start small. Let's say you wake up in a rotten mood and just bear crawl through the day muttering and hating everyone. That would be negative behavior. But, for some reason, around mid afternoon your storm cloud lifts, you feel better, and you start helping, contributing a good attitude, putting away toys, etc. Should you be punished because you only had a 50 or 60% day verses an 80% day? I think not. The key is not to measure up to an inflexible number, because as all moms know, no 2 children are the same. The key is to measure their overall behavior long term and reinforce good habits, positive behavior, and a willingness to participate. That will earn them a reward as a child and later on, prove to be a great discipline for when they become older students and working professionals. I think the key for you is to agree with your husband with a few changes and let him know you are more concerned about long term verses one day of good behavior. Praise them up and down for every good deed and keep reminding them of their goal. For my sisters and I, when we were little, it was a trip to Disneyland. It took us an entire summer to earn it, but we did it. I think I was 10, the littlest was 5, and the oldest was 13. For your kids, since they are small, it can be a pizza party or that toy they always wanted,etc. It just has to be something that isn't par for the course that they are excited to work towards. When they get discouraged, remind them of how great that reward is going to be.
I hope that helps--good luck mama.