D.K.
I've heard that a lot of moms will hire a teenage (or younger) girl from their church or somewhere where they know people well, to be a mother's helper. Most girls would love to have a part time job like that over the summer. I've even heard of some moms hiring a local college student. I think the youngest I ever heard of for a "mother's helper" was about 9yo. If you think about it, it's not so far-fetched. Especially if said 9yo has younger siblings and has already been acting as their own mom's helper.
Try advertising on your local church bulletin board and if you plan to continue into the school year, switch to a evening and weekend schedule and make sure to keep things flexible to accomodate her family, activities and academic/social needs during the school year. Since your children are so young, you have the more flexible schedule. You can go to the grocery store or park in the evening instead of the morning. You can take the kids shopping for new shoes on Saturday instead of Tuesday etc...
Oh and I would think that $4-5/hr. would be a good wage for something like that since you're still going to be around and it's more of an assistant position than the full responsibility of a babysitter position.
Good luck! I hope you find someone great!