Be clear about your lower age limit (typically 18 or 24 months, but sometimes expressed as "toilet trained")
Have a drop off sheet for each child. Collect contact info for the parents, but also information about their developmental stage, like whether they take naps or need help with the bathroom.
Plan activities for the children. It can be as simple as "now it's time to stomp on bubble wrap".
Have a range of different age appropriate toys.
Get one of those Polaroid sticker cameras, photograph the parents, and put the sticker on the back of the child's badge to confirm who can pick the child up.
Have healthy snacks and drinks even if you close for meals.
Don't forget to bring outlet covers and a bathroom stool.
Children's Programming (compare, this is NOT Babysitting)