It doesn't all have to be words either. It can be doodles or taped on images If you're going the electronic route, emojis, photos or other images could be used.
Need some ideas when writing your diary? Here are some books:

by Kinney, Jeff
Can't start with anything but this "classic" ...
Greg records his sixth grade experiences in a middle school where he and his best friend, Rowley, undersized weaklings amid boys who need to shave twice daily, hope just to survive, but when Rowley grows more popular, Greg must take drastic measures to save their friendship.

by Veera Hiranandani
(this one is available on Overdrive right now!)
It's 1947, and India, newly independent of British rule, has been separated into two countries: Pakistan and India. The divide has created much tension between Hindus and Muslims, and hundreds of thousands are killed crossing borders.
Half-Muslim, half-Hindu twelve-year-old Nisha doesn't know where she belongs, or what her country is anymore. When Papa decides it's too dangerous to stay in what is now Pakistan, Nisha and her family become refugees and embark first by train but later on foot to reach her new home. The journey is long, difficult, and dangerous, and after losing her mother as a baby, Nisha can't imagine losing her homeland, too. But even if her country has been ripped apart, Nisha still believes in the possibility of putting herself back together.
Told through Nisha's letters to her mother, The Night Diary is a heartfelt story of one girl's search for home, for her own identity...and for a hopeful future.

by Huftalen, Sarah Gillespie
Twelve year old Sarah Gillespie and her family struggled to make a life for themselves on the Great Plains. Crops and animals needed constant care. Neighbors depended on one another for survival. Through it all, Sarah wrote down her experiences in a diary. Read her story, and learn about the American frontier from someone who lived on it.

I thought my soul would rise and fly : the diary of Patsy,a freed girl
by Hansen, Joyce
This is just one of the many in this series that uses diary format to discuss what life would be like for kids during that time period.
Patsy, an orphaned slave with a bad leg and a quiet nature, is considered slow by the Davis family. But Patsy's smart -- smart enough to learn to read and write on the sly. After the Civil War ends and slavery is abolished, Patsy believes Master Davis's promise to pay the former house slaves and to educate the slave children. But when the master ignores his promise to establish a schoo, and the Freedmen's Bureau cannot provide a teacher, Patsy steps in to teach the students to read and write.