ohhh.... a subjest I am in love with, treadle sewing machines!!!!
lets see to answer your questions
Do you all think they would be good machines for kids to learn at?
YES!!!!
If you have one, do you use it? no I don't have just one, but will not admit how many I do have.... but will say I only use treadles and handcranks to sew on!
If so, are they easy to use? Very easy to use
Is it easy to get replacement or extra parts? yes and no more yes though, if you get a common enough machine and treadle base then you should have no problems, the really old ones and the real obscure ones are harder to find parts for but doable for the most part.
Essentially, tell me anything and everything there is to know about these machines please!
okay you probably do not want to hear my whole spill about treadles and handcranks.... basically most treadles are straight stitch only machines, all metal geared, will last a few lifetimes then some, you can do your own repair work on them, no added costs to take them in to the shop. some of the machines do use odd size needles, some are easy and cheap, some are harder to find and more expensive. If you go with one of the 'old black singers' I'm pretty sure they all use the regular needles most machines today use, (unless you get real-real old singers then a different needle...) attachments are easy to find.
you can treadle some zig-zag models also so if you are wanting fancy stitches then you can treadle and even handcrank them too.
(and I've already started some preliminary research online).
Have you been over to treadleon, great group, people powered machine users, lots of info if you want to use a treadle,
http://treadleon.net/
and to join the email list
http://lyris.quiltropolis.com/script...nter=treadleon.
FW's (Featherweights) are fun and good machines, but there are many other machines out there that sew just as nice if not better and a whole lot cheaper too.
to comment on otheres posts,
for a treadle belt, go check out the ongoing business listings on treadleon, can most likely find belts cheaper from dealers there
parts and feet... the old black singers are low shank machines, so any modern day feet that work on straight stitch settings will work.
I'd stay away from the repro FW"S, not as good quality.
what machine models did you go look at? I could give more info knowing the specifics,
buying a treadle local is probably the way to go, shipping can be very high and so is possible damage to machine/irons if they are not packaged correctly. ebay is okay if you can do local pickup
price is a tough one, the machine/irons do have value, but it really depends on a whole bunch of stuff, it boils down to, how much do you want to or are willing to spend? and where you look, if you go to a high end antique store and looking for a completely refinished cabinet with a machine in pristine condition then you will pay more, if you are willing to clean up the machine or the cabinet then look to pay less. Some folks ay to buy treadles for the cabinet, machines can be easily swapped. If you go with a singer treadle then the hinge spacing is the same so you can stick many different model machines in there, so that opens up so many more possiblities.
something else to think of with a treadle or handcrank, they are so much less noisy so they do not disrupt things if you are in the room helping one child sew and another do something else.
depending on the machine too, it is possible to have a machine in a treadle base, add the handcrank attachment and use it both ways without having to take things apart each time you wanted to switch around, only problem there is sometimes the machine will not lower into the cabinet but if your okay leaving the machine up all the time then things are fine, would be nice for the younger ones who cannot reach the pedal to use the crank then you and the bigger child to treadle
(I think perhaps it'll be difficult for them to reach the treadle until they're older though).
can you make a deal with your cousin? Maybe she'd like a newer machine? 
