Regardless if you consider this a "business" or not.... the parents of whomever you babysit, will or may... claim you on their taxes.
Thus, you need to, too.
And if you put up an "Ad" somewhere advertising your "babysitting"...then someone might see it, and check into that if you are licensed etc. and have your business license etc.
You need to know the regulations in your city.
And per how many kids you may be watching in your home, in conjunction with your own kids.
There is a certain ratio.
Ditto Cheryl B. below.
Parents, who want babysitters, want a regular fixed schedule. Otherwise, they have to find back-up Sitters as well and/or amend their own schedules. So it may be a hassle for them, to "hire" you. Not consistent enough of a schedule. And parents typically want to know, your schedule of what you do with their child and activities/naps etc. And per the age of their child.
Parents also, can often be late, in picking up their child from a sitter. So that will or may, put a kink in your own family's schedule of when you need to leave the house to take your own kids places.
And if you are transporting a child you are babysitting, then you'd need extra car seats. And a "permission to transport" signed by the parent and a release of liability etc.
Your personal schedule of when you can babysit or not and per your own timelines... does seem not conducive to regular babysitting.
Perhaps only drop-in care. For a couple of hours a day.
You need to decide if you are charging hourly or a flat rate.
And have a "contract" to have the parents sign off on, and per any rules you have as a Sitter.
I used to Babysit.