Rhythm, rhyme and repetition are very popular (eg the Bear Hunt: "swishy swashy we're not scared etc" the Elephant & the bad baby "rumpeta rumpeta all down the road", most Julia Donaldson) things to count (eg hungry caterpillar), pictures with things to find in them.
At this age, my DD loved animals, and stories featuring animals. She also liked stories featuring other little children going through the challenges she was facing, such as pre-school. No surprises there I guess. I think the challenge for children's authors is to be able to come up with a fresh take on these subjects. Good luck!