My kids are about 10-12yrs ahead of your kids in age. I think that you are probably about right in your expectations for their age. They are just getting to the age where they can be helpful on the farm. I have always felt that I should not pay them for house keeping chores for two reasons: 1) When they are adults no one will pay them to do their own housework 2} It isn't a tax deductible expense.
Farm work is a different matter. Shortly after the age of your oldest we started paying ours a salary. The salary didn't help the work ethic that much but our kids were never complete bums. By the time they got in their mid teens we paid them for loading pigs by the load and barn washing by the pen and vaccination by the pig. The piece work payment really woke up the work ethic in them. They get up at 4:30am to load pigs 70 mornings a year and they compete to wash the most pens. They do field work and a little bit of cattle work t earn their salary.They have to save 1/2 of their income.
Sometimes getting kids to work and be responsible seems like it is more work than what its worth, but that is only in the short term. In the long term good kids are your most important work.