First off, cheers to you for making the trek. I hope your family's experience in Ames is a great one, and you can enjoy yourself here even though the Cowboys leave with an "L".
If I were you, I'd take the girls to Great Plains Sauce & Dough for pizza, or if you're looking for something lighter you can grab a sandwich from Downtown Deli, then take a walk down Main Street and check out the local shops and eateries.
Stop at Cafe Diem for a coffee or hot chocolate, Chocolaterie Stam for some sweet treats, and then either Della Viti for a glass of wine, or go slightly off Main Street to check out the arcade bar Time Out Ames. It's kid friendly until 7:00 p.m., and has a ton of arcade games, pinball games and skeet ball, for no more than $0.50 a play for most games.
There are also plenty of places to shop on Main Street, including Duck Worth Wearing (kids consignment store), the Loft, and Cooks Emporium. Go to the Vinyl Cafe and check out their selection of vintage records. Plenty of other options as well:
http://www.amesdowntown.org/shop-1.
As many have said, Hickory Park is a local legend, and while the food is good (depending on who you ask), the kids will love the ice cream treats. The wait can be insanely long, especially on game day, so be aware of that. If you're looking for good barbecue, I'd skip Jethro's and go to a new place called Cornbread BBQ, which is at the West end of Main Street. It's brand new (formerly a food truck) and is pretty terrific.
As others have said, the other "kid-friendly" spots would be Perfect Games, as well as Reiman Gardens which is right next to Jack Trice.
Take a walk around the Iowa State campus, including the Memorial Union and the Campanile. I'm biased, but it's one of the most beautiful college campuses in the country.
If you can, find yourself some fellow Cowboy fans, or better yet a friendly ISU group to tailgate with. It's not game-day in Ames unless you find one of these. ISU fans are known for being hospitable. There are always going to be some bad apples in the bunch, but hopefully you can see past them and find some good ones. Football in Ames is special, and our fans know how to do it right.
Hope this helps, good luck and have safe travels!