Roller shutters do not darken - really no reason for complaint?

Yes, but even if the rails don’t go all the way down, the blind doesn’t stop there but continues down. It practically runs out of the rails.

The distance at the bottom is completely normal. The rails usually also stop about 1-2cm above the windowsill and the straight slat always extends about halfway out.

What can be seen here is simply a roller shutter curtain that is not wide enough, allowing light to enter at the sides. The gap at the bottom could possibly be closed by programming new endpoints. At the sides, it will likely remain as it is. I see no chance for improvement there.

New roller shutter curtains are not very expensive. If it really bothers you, it can be replaced. However, the child's sleeping behavior will probably not change much because of it, or the child will eventually sleep through the night even with this marginal light source.