For your 3-day Snowdonia trip in mid-April with a 13 and 9-year-old:

Day 1 - Underground Heritage: Start with Llanfair Slate Cavern near Harlech, offering self-guided helmet and torch tours through historic slate mines suitable for both ages. This provides educational value and indoor shelter if weather is poor.

Day 2 - Scenic Exploration: Take a mountain train ride through Snowdonia for relaxed sightseeing, then pursue outdoor activities like family walks around lakes or easy trails. If weather permits and children meet height requirements, consider toboggan runs where your 9-year-old could potentially ride with your 13-year-old or an adult.

Day 3 - Coastal and Castle Adventures: Explore medieval castles in the region or head to the coast to visit Llanddwyn Island Lighthouse. Beach walks offer scenic beauty, especially near sunset.

Special Note on Go Below: This 5-hour abandoned slate mine adventure with boating, zip-lining, climbing, and abseiling is excellent for your 13-year-old (well above the 10+ age requirement). Your 9-year-old may not meet minimum age/height requirements, so verify directly with the operator about exceptions or alternative participation options.

Weather Planning: Mid-April typically brings wet, wild, and windswept conditions. Pack layered, waterproof clothing and plan indoor backup activities (cafes, museums) for rainy periods. The season offers early spring scenery with morning mist around lakes.

This itinerary balances adventure, history, nature, and age-appropriate activities while accounting for variable spring weather.