These aren’t exactly exploration games, but they’re simple games that my toddler likes too:
Animal Crossing is easily her favorite. She loves “helping” my wife pick outfits and fish.
A Building Full of Cats is short, cheap, and cute. She likes making up stories about each apartment and cat. There’s also tons of similar games in different locations.
Cats in Time has simple puzzles that she can do with a bit of help.
Slime Rancher might be a good fit. It’s simple and cute with a focus on exploration.
Dorf Romantik is a relaxing and cute game that’s a good introduction to resource management. She might not be good at the actual goal of the game, but she likes placing tiles.
Subnautica in creative mode might be interesting for exploration, depending on how sensitive your kid is about some of the darker areas and creatures.