My son came with me for the first time when he was 2. Sat a lot long that I anticipated, but still, it was a short trip.
Patience is the biggest thing. Every single bob of a bobber means he sets the hooks and reals it up. We go through about 4 more scoops of minnows than needed because he's constantly taking them out of the water and putting them back (or not and then they wind up lost in the shack, only to be found at season's end as mummified minnows).
Last year I sprung for an underwater camera. That was pretty fun and added another edge of excitement. But he's still not there to sit and just watch a bobber.
I started bringing a heavy wool blanket and his tablet. I'm not one to use electronics as a crutch, but if it means actually making a trip out of an ice fishing day, I'll let him watch for a while. The rule is the tablet does NOT go past the bench. So he lays down in the back of the sled wrapped in his blanket, watching a movie while I fish. Eventually if the bite picks up he'll put it down and reel in a few. But I found this to be the best way to lengthen a day. If it's nice out I let him run around on the ice and explore. But for some reason the past few times we went out were pretty darn cold and he didn't last long. The worst thing that can happen ice fishing with a kid is them either getting wet or cold or both.