I make I spy quilts in a more sophisticated collection of fabrics for senior citizen quilts. I think any age is fine, but you might want to use some preschooler appropriate novelty fabrics mixed in -- maybe some super heros, household items, sports stuff, foods.
You can make it even more fun by printing up a tag with a list of items to look for. IF you have a lot of novelty fabrics, maybe you can make it an ABC search -- items to represent every letter of the alphabet.