I definitely wouldn't try to walk from the airport, nothing worthwhile anywhere around there. Unless you like cargo warehouses and factories.
However, SEPTA Regional Rail Airport line has a stop at each terminal and connects into center city. From there, it's pretty easy to use the RR or the tourism Phlash bus to get to all the interesting bits.
For convenience I'd go with the Marriott airport hotel, so you can walk to terminal B for check-in and bag drop. Don't know if you have any Marriott FNCs but a 35k cert usually covers a night (30-35k Bonvoy pts a night when I've booked) Then when ready to explore just walk out to the train station at the same terminal. There's many more hotels around the airport for a bit less that have a free shuttle but then you have to schlep bags back and forth. Definitely weigh convenience and time if you decide to come in the night before.
Train takes you to the three main center city stations (30th St, Suburban, Jefferson). Most things are easy to walk to but for potential time savings, the Phlash bus route was designed to hit many popular attractions, including the Please Touch Museum and the Philadelphia Zoo. (Although late August is not when I'd want to visit the zoo but that could be a personal weather preference.) A one day pass is $5 and under 4 is free according to the website. The Please Touch Museum still gets high marks I believe and is geared towards the under 7 crowd. Reading Terminal Market is also usually recommended but fair warning it can be a crush of people. (Personal space, what's that?) And of course all the historical sites - Liberty Bell, Independence Hall, Constitution Center - museums, Penn's Landing if you like water and ships. These are all stops on the Phlash.
https://www5.septa.org/travel/routes/rail-airport-line/
https://ridephillyphlash.com/