We have an IKEA loft bed for our 12 yr old.
One of the beams that prevent legs going apart, is messing up with access to his chest of drawers.
I want to change that beam to diagonal braces like these:
What would be a good way to attach them to the bed frame to ensure legs don't come apart? The legs are 6x6 cm, and the horizontal frame part is 2x10 cm. The diagonal braces I want to use would be 7x4x40cm.
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.
The lateral support on your bed mostly comes from the two connections at the top creating what is called a moment connection (think of lifting a long stick with both hands). The middle members do provide some additional support but consider that there is NOT one on the front and this should make it obvious that they are not absolutely necessary to keep the bed from falling over.
The middle brace is more about keeping the relatively thin and tall posts in place. In particular, the BOTTOM of the post. The braces are located in the middle for reasons such as aesthetics, to prevent toe stubbing (if the middle piece was not there then people would walk through and hit the bottom piece). They do provide some additional support against buckling but that is minor, again note that front piece does not exist.
My suggestion is to prevent the bottom of the posts from moving. If you have carpet then it's probably fine as it is as long as no one tries to move it. If hardwood floors or for the case of people relocating the post or bed, you can place a cross member in a different location, perhaps at one of the shelves so that it is not in the way, or at the bottom of the post, even under the posts, though you would need to rise the other end the same amount or cut down the posts in question
Your plan is basically sound. Long screws, counterbored through the braces, should hold well.
If you can move the removed beam right down to the floor to tie the posts together, that might help.
If you can attach the posts to the wall, that will help some.
And if you discover the bed is still wobbly, a wide plank where the braces are would be my next step.
I built a very similar bed but without any of the lower braces.
I anchored the top bed frame to the wall on two corners (diagonally opposite). This means that there is no side to side movement of the bed frame. So the posts are simply bearing vertical (gravity) load.
Its been rock solid for a year.
When I built my grand kids bunk beds I used 3/4 inch plywood at the foot and head of the beds. In your case, you could put a panel of plywood that went as far down as possible under the top bunk. And it could go up as far as you want past the top. That is what I did so I could add a book and toy shelf to the bed. In this case just attach with standard #8 screws (drywall, or woodworking).
