There are a number of nice 24" smooth tread tires for use on paved roads. You can certainly go a bit narrower like to a 1.5" width, and possibly to 1.25" though that would depend on the rim width. Here's a chart you can use as a guide to compatibility of tires and rims.

I'd probably stay at 1.5". that would provide a bit more freeboard and be more forgiving if potholes and bad pavement, with little or no increase in rolling resistance, compared to a narrower tire.