At that age they should be laying pretty soon. Different breeds get bigger/redder combs and adult feathers at different ages. In general the sex links feather faster so a 15 week old should have enough adult plumage, not to mention they're sex links. Any time I've sold birds on craigslist I put the receipt that showed their hatch date and breed and sex (e.g., all pullets) in the ad, with my address redacted. There should be a fair number of people buying hatchery birds and selling the excess, or go with a more trusted breeder.
I haven't had any problems with hatchery birds, but did get some leghorns that weren't very hardy.
If you get feed store birds (day-olds), you'd have to make sure you got (e.g.) brown sex links, because the yellow chicks could conceivably be white leghorns, or male sex links.
If you mail order, you have to be prepared in case a few arrive dead or die later.
Depending on how big a role the kids are expected to play, you might see if you can find a breeder close enough to you and the kids can help pick out the birds.
I've had a few of the Czech hybrids, and they were pretty calm, especially the amberlink which were developed from that hatchery's ISA browns. In general any mediterranean breed would be pretty jumpy for the kids.