Christine Salerno is like many other single working mothers with small children, but her days are often packed with even more emotional highs and lows: Her 4-year-old daughter, Lily, with soft brown eyes and a wide smile, was found to have Rett syndrome, a rare neurological disorder, last year.

Lily’s case is relatively mild, but she still has limited use of her hands, has difficulty swallowing and can verbalize few words.

Ms. Salerno has spent many late nights researching the types of services and therapies that will help Lily now, but she also needs to think hard about her care decades into the future. “She has 10 therapists and 15 doctors, and I manage all of this,” said Ms. Salerno, 41, of Brooklyn. “This is a lifetime thing. They have reversed Rett syndrome in lab mice. I can be as hopeful as I want, but I still have to prepare.”

Planning for family members with special needs can be overwhelming, particularly when so many decisions may have lifelong consequences. Beyond figuring out the intricacies of government programs, parents fret over guardianships, how governmental services may erode and what legal documents they need.