Well I can answer your question from both perspectives. I worked as a nanny and personal assistant (while doing photography evenings and weekends) for 17 years until I retired (just photography now) to start my own family 3 years ago.
There are several things to consider. do any of your kids need a nanny or are they all in school? If you are going to hire someone to help out it would be more cost effective for you to utilize a nanny instead of paying for daycare as well as an assistant. Also your child would get wonderful one on one attention during the day.
In 1 particular job of mine I was a nanny for 2 young boys. I would generally pick up through out the day (mostly clean up after ourselves). Do some laundry, sweep, vacuum, clean the bathroom once a week etc. Nothing major. My main focus was the boys. When mom came home, she came home to a picked up house, clean laundry and happy well rested boys. I would take the boys on outings in the morning and then clean in the afternoon.
If your kids are school age you might consider having someone run your errands. Do grocery shopping, dry cleaning, post office etc then pick up the kids from school and start dinner. That way when you are off work you don't have to spend more time away from the house doing errands.
Why don't you call me and I would be happy to discuss with you what would be appropriate to expect from your help and what would be a fair price to pay for such services. I can also referr you to placement services. ###-###-####