I had pretty much decided to skip over your question but went back. You might want to consider putting the name of the town in the question. This site is a huge area and the other moms might not want to read it since it is so vague.
One other thing too. You might want to consider there may be pedophiles on a site like this masking themselves as mom's so they can find potential victims.
I would contact the local licensing agency and get a list of licensed child care businesses. I am sure some of them will accept part time child care. Especially home child care. They have been through strict state mandated background checks and are licensed and regulated.
I also think you might want to advertise in a local paper and call it a part time nanny so you are letting someone know they are expected to work in your home and not theirs.
You will need to make sure they understand they are completely responsible for all taxes and other legal obligations for themselves. I worked this way and it is so much easier. I knew going in that I would be filing my own taxes on my income and that the people I was working for would be claiming my payments on their income taxes as child care. I knew I would be giving them my SS# for that purpose. I never had any questions about when I was going to be paid, what my job was supposed to do, how many hours per week, the hours, what I was expected to wear, the family rules (the big ones...like no talking back to adults or going somewhere w/o asking). A good outline of the rules is good to have on the fridge or someplace like that too. Maybe the top 10 and the consequences of them.
I think you could find someone pretty easily though. This age group is not hard to deal with and they sound like good kids. Hope you find someone fun and dependable.