I'm having a rough time buying that a 3.5 year old won't "get" anything in Innoventions. If you take some time to explain what's going on, I'm sure they'll understand what's happening. I know my son did and had a great time here. Geesh, my two year old son even understood the point of Don't Waste It and the Great Piggy Bank Adventure from watching from the side. He just lacks the coordination for the games being that little.

He did great participating in the SlapStick show! Little kids aren't stupid and that age is perfect to teach some this stuff! It's such a nice place to get out of the sun and learn a bit while you play. I say check it out and have fun!
ETA - Our favorites:
Don't Waste It! - Push a garbage truck around and play games to separate garbage, sort items to recycle, etc. My son actually asks to go back to do this one. Your little one probably already understands to sort items for recycling anyways. You can email a certificate to yourself at the end.
The Great Piggy Bank Adventure- Pick what you want to save for (My son picked an Extreme Bedroom Makeover) and move your piggy bank from game to game. Sort coins and stuff. They're fun. You can email the online/home game version to yourself at the end. LOVED this one, but didn't actually try it until he was four. My two year old already understands saving money in his own piggy and loved this, too.
What's Your Problem? - SlapStick studios show. VERY cute and wish we'd have done it sooner. Our boys and then my husband got to be part of the show. I'll save the details. You can make velcro froggies to take home, too.
Where's the Fire? - Needs to be coordinated enough to shine a flashlight on the same fire hazard as you, and then press the button. I thought it was great just for the fire safety aspects and our side won!
StormStruck- They claim this is intense and scary for little ones. My kids weren't bothered and had fun with games after. They're kinda used to a passing storm most days here though. It could be too much for your child.
Test The Limits Lab - This was just fun!! Very noisy. We watched a short video with Pumba and Timon about how to stay safe at home here, too. Who doesn't love these guys? Great info that your 3.5 year old should probably already know!