The going Rate depends on the area you live in, amount of care needed per child. This is a calculator to help figure out what is fair: http://www.care.com/babysitting-rates
You will want to also check credentials for the sitter. Are they Red Cross certified in Infant and Child CPR and First Aid? A baby sitter with better credntials may charge a little more, but you get the security of knowing that your child is going to have emergency care if needed. I found that very important when my son was young.
In my area of Florida, the going rate for two children the ages you have given for 7 hours a day with a Red Cross Certified Sitter would range from $200-$250 a week.
You also need to consider if you have to take taxes out for the sitter or if they will claim to be self employed.
I hope this information is hellpful and you are able to get a good qualified sitter for the children.