I live an hour North of St. Paul so I will give you my two cents about what you and your fam could do here in MN!
1. Mall of America - kids would LOVE the newly-refurbished Sea Life Aquarium (formerlly Underwater Adventures). We have an annual pass and our 3 year old daughter would go once a week if we let her! Also, Nickelodian Universe has plenty of fun rides for all of the ages of your children. There are deals off tickets for both places online; just do a search. You could spend a whole day at the MOA, no problem, and everyone in your family would be entertained.
2. Children's Museum of Minnesota - Again, all ages would be entertained for 2-4 hours. So much to do and see here for your little ones! All hands-on and fantastic! Check out their websites for exhibits and prices...very comprehensive!
3. Como Zoo and Conservatory - inexpensive (free excpet for a suggested donation) and a zoo, perfect for the kiddos! There is a brand-new HUGE polar bear exhibit that is really cool. Go in April or May to take full advantage of this zoo since it is mainly outside.
4. A MN Twins game - you have to take in a game at the new Target Field in Mpls! Amazing! So much fun for all ages! Get some cheap seats and have a ball! (no pun intended) On a nice May day, even a few innings for the little ones is going to be a great experience here! We plan on bringing our 3 yr. to her first game this summer. Many friends of ours took their little ones 2-12 last summer and said it was such a fun family experience. I just love outside baseball!
5.The Minnesota Arboretum (sp?)- check this out online. We have not been there...YET. It is on our to-do this for this Spring/Summer. We have heard so many wonderful things about what it has to offer for kids/families.
A lot of great family places to eat, inexpensive here all around....easy Spring vacation! Don't hesitate to PM me if you want any more specific info. My DH is from St. Paul and knows the way around both Twin Cities like the back of his hand!