We are heading to the St. Paul area next week as part of a two week road trip to visit family, friends and our old stomping grounds. My kids grew up in the southern suburbs of St. Paul, as we lived there for 8 years. We moved away in 2004, but go back every few years to visit.
I'm not sure how long you plan to stay, but a trip to the Twin Cities isn't complete without a stop at the Mall of America, according to my kids:
http://www.mallofamerica.com/home
I'm sure your 4 year old DD would love eating at the Rainforest Cafe there. I also recommend the Minnesota Zoo:
http://www.mnzoo.com/
the Como Zoo (it's free, whereas the Minnesota Zoo is not):
http://www.comozooconservatory.org/
and the Science Museum of Minnesota:
http://www.smm.org/
If you have time to take a day trip, drive south on Highway 61 (along the river) to the town of Kellogg to visit Lark Toys (it's about an hour from St. Paul, give or take). It's an adorable toy shop/vintage toy musem/bookstore/snack bar with the most amazing hand-carved carousel you have ever seen. It's a must-do every time we visit, and when we lived there, we drove down there at least 2-3 times a year:
http://www.larktoys.com/
We also enjoy going to Stillwater to walk around the cute Main Street area.
http://discoverstillwater.com/home.php?gclid=CK6ng9DC7rACFcqe7QodVm1oWA
About 20 minutes north of Stillwater is the town of Center City, and there's a little shop there called Eichten's that has a cute gift shop, a restaurant, and a herd of buffalo. It's another must-do for us:
http://specialtycheese.com/
When we lived there, my absolute favorite restaurant to go to was in Woodbury (a suburb on the east side of St. Paul) called Champps Americana. It's a sports bar, but is very family-friendly. We ate there several times a month for 8 years, and NEVER had a bad meal. Their spinach artichoke dip is legendary, and I'm already drooling at the thought of the "Ellie's Pasta Salad" I plan to have when we go. Delicious!
http://www.champpsmn.com/woodbury/index.php?option=com_content&view=article&id=49&Itemid=54
Hope that helps. Have a great trip.