The swimming pool is part of the fitness center area and very much designed for exercise, so don't think it would work well assuming the kids would want to splash around. IIRC they require adults to wear a bathing cap even (unless I'm confusing it with a different hotel we stayed at in Japan). That said, I would imagine most hotels with a pool in Tokyo would be a similar setup, so I don't know that I would count on finding a kids-friendly pool as part of your decision process.