We'll also be flying over this summer -- too bad we're stopping at Toronto or ours could've played though we're then taking the train to Detroit (for a bit of variety this time).
Going out you'll have a day flight and coming back will most likely be night.
Take some snacks. Airline with seatback tvs are a must. Last time my DD (then 3.5) watched the entire way out. You may want to consider paying for a seat for your DD. As whenever the seatbelt light is on, she will be strapped to you on your lap. Some will provide little seats which fit into the airline seat which mine have always found confortable. Take them on laps around the plane. Most people are really tolerant. Book child meals if poss. Don't take any toys with small bits as they will get lost.
Once you've chosen your flight check out on seatguru for configuration of the plane. Then book your seats. If the plane is a 3-4-3, we usually book the middle four. But this time we're on a 3-3 seat config so on the way back we're booking the 4 seats as 2 lots of 2 (e.g 32CD & 33CD) so that each adult has one child to deal with on the nighttime flight and hopefully they'll snuggle down and sleep.
You'll probably land late afternoon, try and get the kids to stay up until at least 6pm, then (hopefully) they'll sleep through and be set on that time. For the younger one, I just added in extra snacks and try to stick to 3 main meals if that is what they are used to.
I also find using PurpleParking or some similar extremely useful. They take your car from the terminal to carpark and then pick you up at terminal with you car. Great with the kids & all that luggage.
BTW Minn/St Paul is a great airport. Don't forget to go to the Mall of America