Not the page everyone is on, but I'll mention visiting a brand new water park in Bangna, at the Central Bangna, on the roof. It used to be Leoland, another water park, but they closed that for a couple of years to do renovations.

Now it's Pororo water park, based on the Korean penguin cartoon character. The place is nice, a decent alternative for younger kids, around 400 baht for adult entrance for a day, so not so expensive. It's not expansive, just a few larger slides (4), and different depths of pools, and a smaller kids' play area.

http://www.pororoaquapark.com/en/main

small and I would have to be watching a kid constantly in such a place, are there safety staff on duty?

There are people supervising the kids in the different areas but I can't imagine just letting the kids run in a place like that, especially for younger kids.

Then again, I grew up in a different world where you would just let your kids run free in amusement parks when there was some minor risk related to that people generally wouldn't accept now.

As an example of staffing level, that section in the photo has two small slides just out of frame to the left and a staff is there continually to make sure the kids use them properly. All that section with the slides in that picture, and stairs, and water spraying everywhere, is not directly supervised, beyond that staff being within site of there.

Even in the US or Europe there can't really be eyes on your own child all the time so accepting some degree of risk or doing just that yourself is always going to be the two options.