Definitely visit the Aquarium, it's really first class. It can get quite crowded, though. You might want to call or Chat GPT to try to figure out what days are less crowded.
If you have some time to get outside just a bit, Amicalola Falls is a really nice state park quite close to Cumming. There is a really nice waterfall with a hike up to the top and a hotel at the top of the waterfall just off of the Appalachian Trail. If you have more than just a bit of time, I would trek up to Tallulah Falls. It's quite spectacular and it is a bit higher up in elevation than Atlanta, every bit makes a difference this time of the year. It's going to be hot.
I'm not sure where you are staying as Cumming and Marietta are not really all that close to each other when you put Atlanta traffic into the equation. If you want a place to grab a bite where kids can run around and not get into too much trouble, The Collection in Cumming will fit the bill.
I like Delbar, but honestly I like Rumi's better if you are going to consider a nice evening of Persian food. It is a bit pricier, and the menu is probably a bit more limited, but all of the meals are very well done. There is one in The Avalon at Alpharetta, you can send the kids to NFA Burger or Superica while you get a nice meal at Rumi's Kitchen. South City Kitchen is inside the hotel at The Avalon, they serve very good "Southern" food and the bar scene can be lively. And Cafe Intermezzo always has great desserts. There's also an ice cream place, I can't remember the name, in the middle of one of the streets (boulevard style) at The Avalon.
Enjoy.
