Weekday Babysitter Needed - Flexible Hours

We need a babysitter for our 3-month-old son during the work week (minimum of 20 hrs/week). We both work from home so we have some flexibility on start/end times each day, but would like someone who can work at least 4 hours/day. We have a 6-year-old daughter who will mostly be at camp and then at school fulltime in August, but may need some supervision some days as well.