Yes, you need to vent an attic.  Single story house have the ceiling of the room with insulation on top of it, just like your house and then you have venting in you awning with vents in the top of your roof.  The natural up draft of the air takes away the moisture and keeps the roof the same temp (appox.) as the outside temp.  No moisture is built up on the inside of your roof or in your attic. 

I would think if you want a play room in the attic that I would build a room up there.  Insulate around the room on the sides and on the top.  Leaving room for the air to move around the room.  You want the roof to be able to fluctuate in temp just like before.  If you insulate the roof you would be boxing in that attic and stopping that natural air flow through your attic.  I would not want that. 

I am not a builder, just a home owner and telling you what I have learned over my life when I lived in an old farm house and in my 14 year old house today.  We called it a "cold" attic.  It can also get darn hot, in the summer. lol.