My family visited Imiloa as our "Plan B" for inclement weather in Hilo; it rained almost every day of our visit so we chose a few indoor attractions to occupy us on those days too rainy for outdoor activities. It was fun and educational but a bit on the pricey side; $17.50 per adult, free under 5, and $9.50 for kids 5 and older. Discount for seniors, military, and kama'aina. Price includes one planetarium show, which is fun but can be overwhelming for very young children. Parking is free. We did not visit the gardens or the cafe, so this review is for the astronomy center only.
Both of my pre-K aged children enjoyed the astronomy center; we spent about 2.5 hours here. The older one (5 years old) got more out of it; the younger one (3 years old) did not understand how to interact with many of the displays and was frightened by the loud chanting in the planetarium show. You may want to skip the planetarium show for very young children. Adults and older kids will likely enjoy it. The free "4-D" short film (7 minutes) with 3-D glasses was VERY cool and my kids wanted to see it twice.
TIP: Use the coupon you can find in the Hawaii Visitors brochure at any of the major tourist attractions or at the airport for a small discount on admission, as it is a bit on the pricey side. Or, visit the website to learn the Hawaiian word of the day for a small discount on admission: http://www.imiloahawaii.org/169/WOD-discount. Recommended for adults with an interest in astronomy for families with children ages 5+, particularly on rainy days when outdoor activities are less attractive.