I did a Lyman Plains Muzzleloading Pistol Unassembled Kit 54 Caliber Percussion 9-3/4" Blue Barrel $237.99 a while back. Blueing the barrel was a non-issue. I got it right the first time and I have never blued anything before. I was in a hurry to get this shooting and probably didn't do as much sanding on the wood as I could have, but I like it. If in the future I decide I want to sand it some more I could.
It will take a total of about 24 hours to go from box to shooting, but several of those hours will be 1 hour at a time applying blue or some type of finish to the wood, then waiting 24 hours.
In the end I had a companion to my Cabelas 54 Hawken. For the kids I suggest NOT shooting full house loads out of either as full house load KICK. Stick with moderate loads of patched round ball.
All in all, it is a fun project, BUT if this is their first gun I would suggest looking at the long gun. It is more expensive, but they get more out of it. I believe they could earn a shooting badge with their own long gun. If they are inclined they could hunt with their own long gun...... I don't think it would take that much more time.
Darwin, where are you located? If you are local, I could let you play with mine, have some cold blue....