how can I progress this session?

ADD AN ADDITIONAL DEFENDER TO THE CENTRE GRID, OR CONDITION THE CENTRE GRID, SAY LEFT FOOT ONLY, 2 TOUCH ONLY OR PERFORM A WALL PASS BEFOR IT CAN COUNT AS A POINT

Condition your players, to one touch/two touch. Condition your wide players that they must make so many/so few passes before crossing the ball. Overload your central area. Split area into three (attacking third, central area, defending third) overload if required, so many passes in each third prior to ball going wide for the cross. play with four small goals at either end instead of two, which makes the wide players think more? Be creative & innovative - Ask the players opinion you will be surprised? Remember player/coach is a two way thing.   

You could post the wide players along a line from which they cannot move infield, so they can move along the line. This makes it more difficult for the inside players to reach them but easier for the wide men to cross.

You can also post one 'free' player in the middle who only plays for the team in possession.

You can decide that the first touch is always with the weak(er) foot (also in addition to what other posters said).

Furthermore, you can decide that all balls cannot go higher than knee-height (or all balls in the middle part cannot go higher than). This places more emphasis on movement off the ball.

add in another goal and make it a 3v3 in the middle area and it becomes a game.  5 consecutive passes out to a wide player, cross and finish.  just like the wingers game really but with stipulations, you could do 3 passes out wide and focus on a swith of play through the middle area again before you can attack and cross and finish.

you could make it into zones that when the crosser gets the ball only 2 players can attack a defined area and only one defender (if you are looking for success going forward) or more competition 2v2 in the attacking zone...whatever you decide or whatever you are wanting to achieve from the session.