Time for hubby to get with the program for the next 18 years or so: You vacation in places and in ways that work for the KIDS. If the kids are happy, you will be happy. He has to learn to take pleasure in seeing THEIR pleasure. And not traveling at all will deny the kids a lot of chances both to have fun and to learn. Yes, adults need their grown-up time and pursuits as well, but that doesn't happen when away on vacation, unless perhaps you stay with the kids once they're in bed and he goes out to a movie or whatever. Oh, and he does the same for YOU.
I bet some of his stress is having an infant along and yes, it does require more gear that if it were just the two older kids (car seat, diapers, etc. etc.). But it's doable if you're realistic and if he just relaxes about it.
Meanwhile, your six-year-old is at a perfect age for Chicago. The science museum is wonderful--pure heaven for a kid this age. The planetarium is right next door to it. And the seven-month-old isn't walking yet, which is terrific; the baby stays in a stroller with one parent in charge, armed with tons of baby books and distracting new toys the baby hasn't seen yet, while the other parent goes off through the museum with the six-year-old. Families with kids of such different ages simply have to "tag-team" it. If you don't, the older child is always hearing, "We can only stay a short while, the baby will get tired...we can't go in that dinosaur exhibit, the sound effects are too loud, the baby will be scared...we have to leave now, the baby's crying" etc. Imagine how much fun that is for the older kid. And how many learning opportunities the older kid would miss. You and the hubby need to be willing to take things in turns with both children, and that can even include one of you hanging at the hotel all afternoon with the baby for baby's regular nap schedule, maybe some by the pool time, etc., while the other parent takes the older kid to...Go up the Hancock Building to the top for the views, go to the children's museum that's by the lake, etc. etc. You definitely sound up for this, but you may have to convince your husband!
There's a "Chicago card," I can't recall the formal name. It's a discount card on lots of museum entries and other tourist destinations in the city. You pay for it and it's hefty but it saves you money in the end if you actually use it wisely. Check into it. And with hotels --better to pay more for a good city-center hotel that's minutes from stuff you want to see, than to save cash on some far-away hotel at the airport etc. and spend too much time getting into town and back every day. You want to be able to send one adult "home" to the hotel with the baby quickly and easily whenever you want or need to.