Old House Attic Flooring

I would like to put some plywood sheeting down in attic so my kids can hang out up there ( no big furniture). Checking to see if anyone ha done this before without having to sister joist etc (pics below)

*looks line 2X6 16" center..span is about 9ft on either side of loadbearing wall
*I have plaster ceiling below
*what size sheeting?

thanks or any advice

Rich