I have been obese most of my life. I am finally as an older adult learning to eat correctly. However, I taught my kids to not do what I was taught. As adults they are in the right weight category and very active. The clean your plate mentality was good for America when food was hard to come by and people worked in fields all day. As your kids become adults they will keep that eat it all even if you are full habit, and will suffer weight issues I promise you this.
If your kids are full at dinner and did not eat it all do not make them over eat. However, if in an hour they are wanting sweet treats then they are really wanting that sweet treat. Sweet treats should be limited to special occasions, and weekends. Knowing you are adding sweets to the rotation for weekend meals watch the fat and calorie content of that weekend meal.
Most of us do not consume enough fruit in our day. On weekdays feed them dinner and before bed if they are hungry give them fresh fruit, not canned. This will set up an excellent life time habit. If they do not want the non sugar food for late night then they really are not hungry are they?
Also, saving popcorn and sweet treats for the weekend makes it more special, and can be great family time.
I would start of serving smaller portions for sure, making sure that they are getting in all the veggies and not just eating the meat and saying they are full. Also this might help hubby by them eating all on the plate issue if the plate starts off smaller. Regardless I would give him some literature on how children should eat, to help retrain his mind. Don't worry many of of have had to retrain. Many of us need to and don't, just look at wal-mart and you will see how we are not getting it yet.