A book that fits the reader is easier to finish, and finishing is what builds confidence. Age is only a starting point. Attention, vocabulary, and mood matter just as much.
Toddlers, about ages 1 to 3
Board books and sturdy picture books still win at this age. Big faces, animals, and lines that repeat give a toddler something to point at and say back. Bedtime, family, and everyday routines are enough plot.
Preschoolers, about ages 3 to 5
The stories can get funnier and a little longer. Friendship, a small problem, and a clear ending keep a preschooler with you. Many of them like to guess what happens next, then tell the story back from the pictures.
Early readers, about ages 5 to 8
Decodable readers, illustrated chapters, and the first book in a series are a good fit. Let them finish short books before you push the level up. A finished book is worth more than a harder one left on the couch.
When you try a new ebook, read a few pages together first. Laughter, a question, or "one more" is a reliable sign you picked well.