If you do want to have them start some sit down, workbook type activities(most young children love doing these in small amounts)the Dollar Tree has a ton of workbooks geared towards those early years. If you don’t have Dollar Tree near you you can also place an order online.
And I double the library idea, it’s a great resource, be sure to become buddies with the librarians, they will often give you the heads up on preschool events, etc..