Living in the area, I go to Great Mate every Thursday morning, it is pretty much open all day (apart from the lunch break). I go in the morning because my daughter likes the story time, but otherwise it is free play. True it is only open on Thursdays.
Another place to check out is SKIP- closer to Sai Kung town, which has both indoor and outdoor space. They will have summer Paint & Play 2 hours sessions every day. You need to buy tickets for the session.
I hope it helps!