Hungry Hungry Hippos - if you're OK losing all the balls after about 3 times. Candy land they can get, although it's pretty boring. Mr bucket - buckets of fun! Except for your back. Be prepared to lose if he can't deal with it. That's what makes it boring.
Mine have been going hard on Candyland. It's become a daily thing that they've been doing for almost 8 months now...one of the few things that they haven't lost interest in. We also do a version of go-fish but we actually show all the cards just to get them practicing the visualization of it. Might try to play a proper game soon. The Ring-a-Ding game is fun but they lost interest in it pretty quickly. Count your chickens has been okay as well, but they always go back to candyland
So I'm guessing Connect 4 is meant for older children as these two are older than 3 and are still playing despite green having already won.
I think stacking is a mechanic that a kid that age can start working with their dexterity. Plus it's a niche where the games are packed with color or kid themes. The ones that come to mind that could work with someone that young are Rhino Hero and Animal Upon Animal.
Pretty simple to wire the battery back to the tweezers in Operation. Kid needs to learn that his mistakes have consequences.