I think any breed is ok, as long as it is one with a calm demeanor. Some breeds are known to be flightier than others, most of the Medditeranean breeds. Try and go for a calm breed, and have the kids handle them a lot from new chicks on up.
The pp who mentioned smaller birds for showmanship, gave you good advice. If they are going to do showmanship, they need to be able to hold the bird, spread the wing, flip it over and show the judge the keel bone. Having a smaller bird makes some of these tasks much easier for the kids, especially if they are just starting out. Having a big heavy bird will make it more difficult. So if you are sure you want a LF for your older child, try and find a "smaller" large fowl. And maybe have them use a pullet or hen instead of a roo for showmanship. Not only does the bird need to be used to being handled, it has to stand on a table for a 1/2 an hour or more with other birds right beside it. Sometimes the roosters don't like being next to other roosters and it will be up to your child to keep it under control. Also get your child involved, and let them help pick out a breed they are interested in! Most of all, have fun!