Located just five-minutes' walk from Gong Guan MRT station (behind the Gong Guan market), this is a great place to take kids of all ages. The park itself is really big. There is an interactive museum, hiking trail up to Taipei's first water reservoir, flower gardens, and of course water play areas. You can learn all about the history and science of water in Taipei, or just enjoy a dip in the pools.
Area One: at Shu-Yuan Road side of the park
This is run in conjunction with the water park. At the entrance, there is one free paddling pool that is very shallow. You can go in this one in your clothes if you like, and you can use any water toys you have. It's a nice little place for kids to cool off. Just past this area is the larger paddling pool. Entrance is 40nt for kids and 80nt for adults. You need swimsuits in this area. You can take any water toys you like in there to play with. There is a climbing frame and slide in the middle of the pool. It can get pretty hot as there is no shelter over the pool, so it's good to take sunhats to wear in the pool. Snacks are available.
Area Two: Ding-Jou Road side of the park
This is run separately from the water park. Entrance fee is 250 per adult, 150 for children 120cm or below. This is a huge swimming club, with indoor and outdoor areas, water slides, inner tubes for rent, steam rooms and spa area... Here is a link to some pictures:
The English website is very comprehensive, so I won't bother going into too much detail here. Please check the opening dates, times, and fees before you go, as they are a little complicated. One of the best things about this park is it's easy to access, with MRT, lots of buses, and plenty of carparking right in the park itself.