@Seriouslymole, my DS (15) has been fencing (epee) for a few years but has only started to do competitions since last summer.
I'm still pretty clueless about how it all works but I think generally competitions begin with a qualifying round (called poules) where competitors are put into small groups (around 6 in each group?) and each competitor fences all the other people in the poule in a series of shorter bouts. Based on those scores, some of the fencers from each group reach the elimination rounds, which involve longer bouts. In the elimination rounds, the loser is out of the competition and the winner progresses to the next round, until the final bout between the last 2 remaining fencers.
So every fencer gets at least a few bouts at the poules stage before they might be out of the competition, but beyond that it depends on how far they make it through the elimination rounds. Because of this it's quite difficult to predict how long you might spend at a competition, and there seems to be a fair bit of waiting around at the poules stage because there are rarely enough pistes for everyone to fence at once.
There are usually very strict rules about how much coaching can be done during bouts and how vocal spectators are allowed to be - worth reading up on competition info in advance.
Do you have your own kit or use club equipment? Competition instructions often say each competitor must have 2 blades and 2 wires - DS only owns one of each and relies on club equipment if he needs a spare. Definitely take plenty of drinks and high-energy snacks.
Good luck to your DD and DS, hope they enjoy their first comp!