You will also understand that kids up to school age (toddlers) generally only trust people they're very familiar with, it's a process that takes days, not hours or minutes.
No matter how good the checks and due diligence you have in place are - because the majority of parents will not entrust kids to a stranger at the drop of a hat. As codegeek said, trust takes time to build, especially so between the child and sitter. In the process of putting my 2 year old into daycare it wasn't just a matter of dropping them off - there was a month long process where either I or my wife were taking time off work to introduce her to her new carers, making sure she was comfortable with them etc and that was after vetting 4 different care centers.
With sitters, we'd only allow grandparents or my wifes older sister who has similar aged kids too.
[Edit: 2nd para - putting myself in my own kids shoes]