Překlad:Oni si hrají.
"Hrát" is usually used along with an object, for example when playing games (on hraje hry) or musical instruments (on hraje na klavír). But "hrát si" is almost never used with an object and usually refers to children that are playing. The exception would be "hrát si na (někoho)", which means "to pretend to be (someone)".
I think the best translation for "oblékám se" is "I'm getting dressed". As for "oblékám moje kalhoty", people usually don't say it like that and say "oblékám si moje kalhoty". But you don't have to bother about the simple and continous tense, they are not distinguished in Czech.