It's high up and yet it's doable with kids! We're talking about Hehuanshan North Peak. Wait for dry weather (ideally) and start from Xiaofengkou Second Parking or the smaller parking lot right next to the Hehuanshan Service Station building. From the trailhead, you'll start off a little rough and be helped by stretched ropes for a while, then the trail is clear and comfortable in places. We made our way to the summit at 3422 meters and back to the parking lot in about 6 hours with a 6-year-old kid at a leisurely pace. There is nowhere to hide from the bad weather on the way, so you have to have everything you need with you. Plenty of water and food is a must. You will be rewarded for your efforts with unrepeatable views.
Slightly lower and with a much easier route (especially if you like wooden stairs) is the nearby Hehuanshan East Peak. Don't be afraid to bring the kids here either. Plan to start your trail from the small parking lot in front of 3158 Café. You can complete the trail in half the time it takes to climb North Peak. Along the way, you can see the decades-abandoned lift of a former ski slope that has suffered from frequent landslides and typhoons. At Cafe 3158, you can reward yourself after the hike with a sweet but also by buying a cup of this highest cafe in Taiwan!
...yes, it's true. This tip can also be in the "Hualian" section.