If you have a car definitely consider driving to:
1)Forest Park - lots of free parking near attractions including Free Admission Zoo, Science Center, History Museum. (And Art Museum but maybe not something your grandson would enjoy)
2)Botanical Garden
3)Basilica- yes even your grandson may find it cool. And won't talk a lot of time, is free admission. Impressive place
4) Downtown consider eating at Sugarfire BBQ (near the convention center so near the trolley stops)
5)lots of other places to eat unique St louis stuff (frozen custard, other BBQ places, sandwich places,Toasted Ravioli, unique St Louis pizza -not everyone is a fan-)
6)Don't forget to visit Union Station itself - check out the "whispering arches" and laser light show on the ceiling of the Great Hall every hour after 5pm.
PM me if you want more details about anything.