The Children's Museum and Train Museum at the Depot ... fun for kids and adults!

From experience, two hours stuck on a boat may be a bit much for a three year old.  If it is not too windy, my kids never get tired of playing in the sand at the beach.  If the three year old is very adventurous you can try the alpine coaster.  Otherwise any trail near a river in Duluth can turn into a magical adventure for little ones.

Ditto the trails along rivers suggestion. Amity Creek is easy for kids and strollers, and the old stone bridges are always a hit. The weather's getting a little cold, though.

Enger Tower and/or Spirit MT "Timber Cruiser" if the weather is tolerable..

The Marine Museum in Canal Park is free and was a favorite destination for my two weird children when they were 3 and 6.  Still is, for some reason.  Kids would also probably enjoy the aquarium, but it'd be kind of pricey.

And the Children's Museum is a toofer.  If the kids are bored in there you can go check out the trains on the same ticket.  We like the Aquarium as well but the Children's/Train museum is a much better deal.

The Zoo is always a good place to keep children amused while the adults get a chance to walk and talk.

If your friends have a passport membership to a zoo or a museum, they might be able to get into the aquarium or childrens museum free.

A romp in the skywalk system is fun for mobile kids on cold days. 

Also the Christmas parade is friday night 630pm.

Bowling is fun -- the places that have bumper bowling make bowling possible even for wee kids. I think the place downtown has this little ramp that you set the ball on and the kid pushes it and it goes right down the lane.

I don't like the Duluth Zoo, the animals don't look well and the cages are too small. I took my kid there once when she was small and it only depressed us. We do, however, still go on occasion to Animal Allies to play with the cats there. We call AA the AA hotel & spa.

I want to not like it because it's so clearly designed to separate parents from their money, but my five-year-old nephew loves Adventure Zone in the Canal.  There's a playset and huge inflatable slide for toddlers as well as coin-op games and mini golf for the older one.

What is the best taconite picking spot?

Sure don't seem to hear much about the aquarium anymore? I've not been there for two years. Last visit it looked in disrepair and dirty. Is there a reason to go back again? And it was mentioned that it's expensive? Did they raise the prices? I recall it as cheap?

Also the Toddler Tiki time at the Edgewater water park is a good way to spend a cold morning with the kids.  You would need to call and check for sure but I think that $13 for each kid/adult combo for 8-noon weekdays.

For me to willingly endure The Edge would require an additional expenditure at the liquor store.

Last time I checked, the children's museum didn't have anything special on exhibit. At the moment, it's not somewhere I'd take guests. Of course, you get into both if you pay for the train museum, and kids like to climb into the trains.

Bentleyville is fun; advise your out-of-towners to bundle up! You could pack some snacks and hike to the big rock at Hartley, then down into the pine forest. I think the kids would love the aquarium. They'll play with the toy boats for longer than you want to sit there, true, but they like it. There's a slide and tree-fort upstairs. And they can feed the fish at the top of the tall tank. 

If you go out to eat and want to make the kids happy, I hate to say it, but Old Country Buffet is a big hit.

The marine museum at Canal Park would be fun for those ages.   They can go in the mock wheel house and pretend they are captains on the lake.  My favorite beach with my grandson is at Knife River.....lots of small agates to be found.