I have a simple rule for dealing with the children in my life: I never, ever, (shall I say ever once more, for effect?) do what a child demands of me.
Some children have such a sense of entitlement that they view their parents as servants or hand-maidens. They state rather than request what they want (e.g., "You've got to take me to soccer practice tomorrow at 6:00"). Their notion is that their parents' primary role in life is to accommodate their demands.
Parents who permit their children to make such demands on their time and services have allowed the family hierarchy to be turned upside-down. They have abdicated their role as executives of the family and have instead become subordinates of their children.
I recently met with a mother whose fourteen-year-old son told her the previous evening "You need to look over my biology homework to make sure it's right" before going off to play video games. Not only did he demand rather than request her assistance, he didn't have the decency to collaborate with her on the task which was his responsibility.
So, do your children respectfully request your help or do they demand your services? Are the words please and thank you used when your assistance is requested and offered? Do you expect that they request your service with courteousy? If not, I would strongly encourage you to reconsider your approach. In doing so, you can right the hierarchy of the family and resume your rightful role at its top.