I took a 4-yo, a 11-yo, and a 13-yo to the Watermine, and fun was had by all. Bring your own towels, can bring in food. We did it as a double-feature, paired with Reston Zoo, which is right next door. The Watermine is open now (mid-august) from 12 noon, so we did the zoo first, then lunch, then waterpark. Spent 2 hours in the water. The kids loved it. Lots of lifeguards is a plus, but for enjoyable time with small, non-swimming children, I would recommend a 1:1 adult-to-nonswimmer ratio. They have a kiddie pool, too, but the little ones need to be supervised 100%. My 4-yo had his flotation thing on, and was able to really have a great time in the lazy river, and in all areas of the pool, with me next to him. The water is only 4' deep, at its deepest, but still too deep for him, so a flotation device, and a watchful eye from me were necessary. It was not at full capacity, since we came on a weekday, and just as they opened. We had lots of room and no lines. I bet it does get pretty crowded, though, since it is rather small. We paid about $50, for all four of us. I thought, it was well worth the money.
If you own or manage Water Mine Family Swimmin' Hole, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.