Dawn and Whitney, Friends Forever — Ann M. Martin

Dawn gets a special job through the We ♥ Kids Club. Actually, it's not really a baby-sitting job. Dawn has been hired to keep twelve-year-old Whitney company while her parents are out. Whitney, who has Down's Syndrome, is delighted to have Dawn as her friend. Dawn thinks Whitney is great, too. She's warm and fun to be around.

The only problem is Whitney doesn't know her parents are paying Dawn to be with her. And what if Whitney finds out...?