Changing loft bed's beam to diagonal braces?
I'm really concerned about the stability/safety of what you're proposing. The diagonals seem under-sized to me but even with heavier diagonals I don't think the bed verticals would hold up. A 12 year old clambering up and down that ladder is going to put a lot of lateral stress on those vertical supports without the horizontal strut..
Have you considered moving the horizontal strut on the left of the picture all the way to the floor? That would give the stability you will need.
Also if there is a way to attach the verticals to the back wall - better yet.
Edit
Your idea of moving it up instead of removing it is much better. I would keep it as close to the center of the verticals as you can. Again - if you can attach it at the top to the back wall would be best. If you did that I think the diagonals properly attached would work well.
Second edit
If you want to do diagonal struts I would through bolt them with no more than 1/4 inch diameter bolt and washers. With only 6 cm width to work with you don't want anything larger than that.
However, I strongly urge you to attach the posts and/or preferably the bed frame to the wall and not rely on diagonal struts. That's the solution you need. Put wood spacers between the frame and wall and attach it into the studs with  Timberlok screws. All you need is two attach points through the frame. They are easy to install and remove and when you get rid of the bed at some point you only have two 3/16" holes to patch in the drywall.