grand daughter cries when I leave. HELP

I'm watching my grandchildren while both parents are at work. Grandson 6, Grandaughter 3, I get to their home early in the morning to get my grandson off to school, while he is at school I'm with my grandaughter all day. She goes to preschool for 2 hours,twice a week.I watch them until their mom gets home around 4pm. When she gets home it's my time to leave, but this is where the problem is...my grandaughter begs me to stay for a little longer,which I do, then she hangs on to me, and cries now stop begging me not to leave. It's awful to see her cry so much and I don't know what to do to stop this behavior. When I finally leave the house she is in her bedroom window watching me leave and crying and of course I get so upset I start to cry. Please if anyone can give me advice, I would be so Thankful. Thanks for taking the time to read this and any and all your help would ge so great.