Some other (more helpful) tips would be:
- Set a timer, and keep lessons short. They may complain less if they know they only have to work for 20 minutes on that subject.
- Have them do half a lesson per day.
- Turn on some cheerful music to change the atmosphere while they work.
- Work near them, so they aren't working alone.
- Plan a special craft, snack, or activity for after school work is completed. Use it as a reward for work done well, and cheerfully.
- Shake up the routine a bit. If math is becoming a problem, then set aside the textbook and play math games for a day, or two.
- Make learning fun! Turn the day's lesson (or test) into a game or activity. Carol Barnier's book (which I've mentioned before) How To Get Your Child Off the Refrigerator and Onto Learning, has great ideas.
- Take a break. Take a day, or two, or even a week off from lessons.
- Set aside the lessons for just part of the day, and play a board game, or take a nature walk, or go on a field trip with them.
- Go outside for some P.E. or a short recess, then return to the bookwork.
- Set aside bookwork, choose a quality living book, and read aloud to them.
- Keep smiling, and don't take their reactions personally.
- Remove privileges. Work done slowly, or poorly, or with much complaining could result in the loss of other privileges, like TV or computer time.
- Find a new way to use that curriculum for awhile.
- Ditch the curriculum if everyone is dreading it, and it's more than just a discipline issue. We've done this before, but we had a backup curriculum to return to. Perhaps you can find a free curriculum to use online for awhile.
- Come up with a silly consequence for complaining. If you complain, you have to pay Mom a nickel, or sing a song you absolutely hate, or do 5 pushups. You get the idea.
- Have them look up Bible verses on diligence, complaining, obedience, etc.
- Have them write an essay on why they shouldn't waste time with complaining.
- Put some humor into the moment. Sometimes humor gets the point across better than anything else. So, give an exaggerated imitation of their complaining. Exaggeratedly complain about your own chores. See if you can make them giggle, and then remind them that we are told to "Do everything without complaining or arguing" Philippians 2:14-15.
- Memorize Philippians 2:14-15 as a family, along with other verses related to diligence, cheerfulness, and obedience.
- Don't be drawn into an argument with them. Keep your own attitude light and cheerful as you point them back to their work.
Hopefully, some of those ideas will help. Honestly, we take breaks when it becomes too negative. Better to extend your school year a bit than to destroy your child's love for learning by battling with them about their lessons. If we can't afford a complete school break, then we switch out the problematic curriculum with something else educational, but fun, for awhile.