5 hours layover is rush because you need to get back to the airport two hours before and the commute time to Frankfurt city center is about half hour one way so you really only have about two hours to tour around. For two hours, you can either take a taxi or take the S-Bahn to Hauptwache and walk along the Zeil street which is the shopping district, walk toward the old town Romerplatz and then walk along the river you see a lot of historic buildings and many museums on the way. If still time, you can walk over the bridge and see the skyline of Frankfurt...

Not sure how young your kids are, may be too much working for them then you can pick a nice restaurant, probably in the old town square to relax and people watching.