It's not too bad. By the time the rest of the band is also not sounding like a sick animal the trombones have figured it out too. I'm not sure if it's something about the brass tone being more forgiving, the volume of audio coming out of the instrument, the fact that the slide is much physically larger than the violin or viola making it easier to make fine adjustments (and now that I think about it, when I think "the middle orchestra sounds pretty bad" it is mostly the violins and violas, so that's plausible), something else, or some combination of all of the above, but it doesn't take too long (relatively speaking) before the trombones are in tune.

A bad embouchure can put pretty much any brass instrument a full semi-tone out of tune. Trombone is not noticeably different in a beginner band. In my experience it's the beginner French Horn that's usually most out of tune.

But brass being out of tune is not as hard on the ears as the squeaks from a beginner clarinet or saxophone...

Probably the most parent friendly is the flute. It's really hard to get good volume out of a flute so beginners are really quiet and inoffensive. :)