Never too early! Both of my kids started at 2 years old in Alaska. Gotta make it fun so they want to go again. Weather is a big factor, but can be mitigated with a tent and heater. If you can fish by a boat ramp, bring an extra sled for the kiddo to zoom down in when the fishing slows, or when they need a break. Snacks, snacks, and more snacks (all of their choosing) makes it a big treat to go with dad on the hard water! I've even bribed with letting them watch Grumpy Old Men on Netflix on the phone to stretch the day a bit longer (shhh, don't tell mom!). If you have electronics, let them play with them! Teaching them the flasher with a big old jig can captivate them, and an underwater camera is always a hit! If you fish shallow, bring baked egg shells to sprinkle down the hole and illuminate the bottom. Live action fish swimming below them really gets them excited! And last, but probably most important, is the extra clothes that everyone else already mentioned. Keep expectations low, energy high, and time on the ice limited to the kid's schedule, and you will mold an ice fishing monster! My daughter wants to do an ice fishing campout this year (she just turned 9), and is a hardwater fanatic now!