Slicks, and maybe a change in freewheel to give her a little taller gearing. I put street tires on my daughter's 24" right from the start, and it helped, but the gearing is still awful low, so I recently changed the rear wheel, choosing a quick release and a freewheel that went down to a 12 tooth. Helps some, but I really am hoping to get her on 700s next year. The effort is there but the results are still limited by those 24" tires.
Sounds like our riding is a little longer than your ambitions, so you don't need to get too carried away. We regularly do 25 miles and are planning a century the first weekend in June.
"It hurts so good..."