BBJ

I'm just a dirty lurker : but I saw your post about your upcoming vacation. I grew up in NW Indiana and now live in South Bend so I thought I would chime in on things to do.

Beach/Dunes: if your hotel is in LaPorte I recommend the Indiana Dunes State Park. If you take the tollroad west out of LaPorte toward Chicago it is only 2 exits away off the Chesterton exit. It is one of only 2 beaches in the area that have lifeguards on duty.

Mt. Baldy, a huge sand dune, in Michigan City is a big tourist spot as well and is 5 minutes from the Lighthouse Mall Premium Outlets. The Michigan City Zoo is right across the street from the outlets on the beach in Washigton Park. There is a lookout tower you can climb there that has an amazing view. The zoo is small and cheap and great for smaller kids as you can get thru the whole thing in 2 to 3 hours. There is also a lighthouse at Washington Park and access to the beach.

Food: A good family place would be Redamak's in New Buffalo it is a pretty well known locally owned sit down burger joint. Down the street from that is an icecream joint called oinks that is a must visit if you are in New Buffalo.

Along the lines of zoos. South Bends zoo is pretty good for smaller kids too. Bigger than Michigan City's but not as big as Brookfield in Chicago or anything like that. There is a large playground and splash pad outside of the zoo too! South Bend is about 30ish mins east from Laporte via the tollroad. And is also home to Notre Dame if you or your SO are into college football at all it might be a good place to visit. The campus is beautiful with lots of historic buildings.

There is a ton more but thats all the super kid friendly stuff I can think of off the top of my head right now. I will post more if I think of anything else or if you have specific questions/interests feel free to ask.

Edited to add more thoughts:

Minor League Baseball: Checkout both to see if there are games while you are here. South Bend is closer to LaPorte than Garyopposite directions but not by much. Some people would shy away from Gary b/c its the hood but since your coming from Detroit I think you can handle it ;

South Bend Silver Hawks
Gary Rail Cats

Amish Country: If your interested in that kind stuff Shipsawana spelling might be off on that one is close. Lots of antiques/crafts etc.

There is also places to rent kayaks/canoes but I doubt you are interested in those types of things having DS with you.

Chicago: I don't know how long you are around but a day trip to Chicago is always an option. You can take the Southshore Train from Michigan City or Dune Acres Chesterton into the city. Parking at Dune Acres is free and an adult one way is I think 7.50 I haven't taken it in from here for years but we ride from South Bend now which is further away for 11.50 On weekends 1 kid per paying adult rides free. Under 2 is always free. Weekends it takes about 1.5hrs each way to get from Dune Acres to Millinium Stationthe fartherest Chicago stop there are limited express trains that take only an hour.
If you get off at Millinium Station you can do Millinium Park, Buckingham Fountain, Mag Mile, Hancock Observatory, and Lakeshore Drive area without any other public transport. If you get off at the museum park stop there are all the museums science and industry, field, shedd and solider field. I would recommend the Shedd with DS. My son loves it. I haven't been to Science and Indusrty for along time but I think most of the hands on stuff is too "old" for DS at this point. If you do a daytrip Chicago bumpies can probably help you out with better info for once in the City