. Dear Florian,
I have the Bushnell Xtrawide 4×21, which is likely to be the same as the 4×30 as I've seen someone mention that the 4×30 is actually only about 21 mm aperture.
Kids seem to love this binocular. So that I presume there's no problem with the IPD, but I'll try to check later.
I initially didn't like the 4×21 as it is fixed focus and I am longsighted and the centre is out of focus for me.
However, following two bright planets as they got further and further apart from a close conjunction I was able to follow them and measure the field of the binocular accurately as 18.5°. And because the field is curved I find that the image near the edge of the field is in focus for me, so the fact that the centre is not in focus did not matter. So as long as I keep things at the edge of the field they are in focus.
However, children have a great deal of accommodation and as long as they have fairly neutral eyesight I think these are great binoculars. The 4 x 21 were sold off cheaply at a bargain basement price. I don't know how much the 4 x 30 cost.
It must be pointed out that the 4×21 actually only magnifies 3.5 times, so the field is not actually extrawide.
There are several Bushnell Xtrawide binoculars where the magnification is overstated.
I also have the Chinese 4×22 independent eyepiece focus binocular. This has field of 16.5° and genuinely magnifies four times. They were also available with fixed focus eyepieces. I like this binocular even though the coatings are basic single coatings and uncoated surfaces. The images are quite good and the field of 16.5° is sometimes useful.