Ahhh - a little gardener in the making - stirs my soul! Although I now live in zone 7, I have lived/gardened in zone 4 (New York, Montana), so here are a few suggestions:
General points to keep in mind - Kids are drawn to extremes (very large & miniature sizes), bright colors, fragrance, butterfly attractors, and edibles.
Annual seeds you can plant directly in the ground, germinate easily, and are not fussy:
Nasturtium - some are bushy, some trailing/vining, all
bright with big round easily recognized leaves

Sunflowers - seed catalogues have some great colors.
Be sure to plant on the side of your garden where
they won't shade the other plants (usually north)

Sweat Peas - lash some poles together into a teepee
and plant at the base of each pole (smells
heavenly when you crawl inside). Or plant edible
pod peas (sugar snap or snow) - yummy

Bulbs - fun to plant because a child can get their hands
on it -
Daffodils

Allium - a monster purple ball atop a tall stalk.
Allium albopilosum is the big variety that is
cold hardy & will probably survive your winter.
Put along a fence so the wind won't flatten it.

Plants from your local nursery -
Pansy - just so pretty - children delight in choosing
the colors from blooming plants. It's an annual
but I recommend not growing it from seed.

Tomatoes - 'cherry' size is particularly appealing

Highbush Blueberry - if you have acidic soil or
can acidify a spot - delicious & beautiful - check
your local extension agent for the variety best
suited to your area

Bird feeders & bath are wonderful elements in any garden. Your son can fill them, scrub bath & change water himself. We feed & water year round - it's cheap entertainment!
The plants I mentioned probably will not attract too many bees & several attract butterflies. And - bees are nature's perfect pollinators, so a few are a good thing.
Check on the Iowa forum and Butterfly forum too - I am certain that they have some great ideas specifically for your area.
Consider starting him in containers - especially since you are already growing that way. Your son can paint the sides of those big clay pots with designs of his choice. You can paint or spray a topcoat to protect the art from the weather, or just let it wear off and provide a new canvas for more! Lots of the pretty annuals for sale in your local nursery in spring will do well in pots. Cherry tomatoes grow very well in them. Enjoy!!!

Hi DurtyKneez,

Thank you very much for your thorough information!! I was so impressed w/ everything you gave me for info, I just printed it off so DS and I can go to the store next spring and pick some of the items you suggested. I definately didn't think of the cherry tomato plants in a container for him, but what an absolutely splendid idea!! He tends to sneak some from our regular container, so wouldn't that make him proud to have his very own to pick and share w/ ole mom and dad =)
I am really anxious to do this with him next year. I also love the painting idea with the containers so I can pick some of those up either saling or on clearance and we can work on them when it is still too cold out to garden but getting close enough to start thinking and planning it.

Again, thank you again for the wonderful response to my post!!

Stacey and Nathan =)

Stacey & Nathan -
You are welcome. I have had so much fun gardening with my children over the years.
I must apologize - my info on the peas was not accurate. Now that I think about it, when we grew peas (regular & sweet), we wrapped a mesh gardening net around the teepee
(2" or 3" squares). Peas all have little tendrils that cling best to 'strings'. Also - sugar snap and snow peas make more of a bush - the seed packets will tell you which ones climb. Beans, on the other hand, will twist their stems right around a pole. A really fun bean is 'Purple Peacock' - purple when it is ripe. When you drop it in the boiling water to cook, it turns green - my sons got such a kick out of it!
I am so sorry I misled you.
I wish you much much joy in gardening with your son.