A highly-recommended onsen in the Japanese Alps that is open year-round and suitable for families with young children is Kamikochi Onsen. Located in the beautiful Kamikochi area of the Northern Japan Alps, this onsen features family-friendly facilities and offers a variety of indoor and outdoor hot spring baths.
Another excellent option is Shin-Hotaka Onsen, which also has family-friendly amenities and is set amidst stunning mountain scenery. Many ryokans (traditional Japanese inns) in the area cater to families and provide services such as private baths, kids' meals, and easy accessibility to nearby hiking trails.
Both of these onsen provide a serene environment and are perfect for families looking to relax and enjoy the natural beauty of the Japanese Alps together.