A couple of thoughts..

1.) Neither one of those would actually hook directly to the least expensive pickup on Fiddlershop.  I assume you mean this one:

http://fiddlershop.com/electri.....ickup.html

The jacks on both of the preamps Pierre gave links to look too small for the 1/4 inch plug, so you would need an adapter.

2.) I don't know for sure about the "Sound Professional" ones, but the LR Baggs is probably a bit of overkill for just an in-ear monitor.  Not that it is a bad unit at all, but it is more the sort of system that you would want if you needed to supply a great sounding signal out to a mixing board or recording system while playing a violin with a pickup.

Listening with just one ear in a loud environment, I think you may be looking for just being able to hear yourself and maybe protect the ear that the horn section is aimed at.

I would suggest considering something more like this:

qid=1425303663&sr=8-3&keywords=guitar+headphone+amplifier

...since the "Super Sensitive" that is on Fiddlershop is made to plug into a guitar amp. 

You would still need an adapter, something like this:

ie=UTF8&qid=1425304376&sr=1-1&keywords=1%2F4+inch+female+to+female

Fortunately, they don't cost much.

Plug in any earbud you like and you should be set.

The plexiglass shield that Pierre explained would be a great solution, and probably the most deluxe option, if you can talk the organization into going for it.  I have no doubt it would help the other string players as well, while keeping the visual presence for the horn section good.  But they may not be so willing to spring for such a measure "just" for practice sessions.

Fiddlergrrl, from the description, I think that the one you posted the link to is just an output jack since it doesn't actually mention a pickup.  I think that is to provide a 1/4 inch jack for the little mini plug that comes with some pickups that mount directly on the bridge or where they're built into the bridge so you'd be doing a bridge preplacement for.  You'd want to ask Pierre/Fiddlerman to make sure.

The Baggs is a preamp.  No mention in any review I've seen of it having an output for headphones, and the pics I've found only show a 1/4 inch output jack.  It is most likely intended to be between the violin or guitar pickup and an amplifier.  Maybe Pierre has used one with headphones and can say for sure, I don't know.

The Sound Professionals one, I haven't found any info on.  But since it calls it a preamp, it is probably not a headphone amp (which is what you'd need).  Never having seen one before and not being able to find specs on the unit, I can't guess if it has an output for headphones.

Preamps are to get the level of the signal up enough to work with a guitar amp or sound mixing board input.  That isn't enough to drive a set of headphones or an earbud or etc.  

The Joyo.. Sound quality?  I wouldn't expect it to be any better than any small practice amp at that price.  Flimsy?  Well, I suppose it would depend on what they're comparing it to.  Compared to say, a guitar stop box made out of metal, I'm sure it would be.  LOL  It is a lightweight gadget made of plastic.  It is only going to hold up to so much rough use.

I do use a similar little gadget:

qid=1425327623&sr=8-1&keywords=vox+amplug

I've had it for 3 or 4 yrs and use it fairly often for electric guitar.  I don't know as I'd recommend it for violin, since mine is made to imitate a particular brand of amp and I think it would have kind of an odd tone with violin.  Especially an acoustic violin with a pickup.

I don't use a pickup with acoustic violin, myself.  If I want electric, I use my electric violin.  But I understand your situation, at least somewhat.  Not having experienced the volume levels where you and the horn section sit, I can't say for sure if any small headphone device will really be enough. 

That is downright brilliant, Fiddlrgrrl!  A purely acoustic solution, even..

I love it!