How should I fence kids at open bouting as an adult, and is it okay to say no when one asks me?

Fence them with control and a purpose, and yes, you can politely decline any bout, as can everyone else in the room. Open bouting works on invitation, and nobody is owed a bout. That said, turning down every young fencer is a loss on both sides: kids learn distance, patience and tactics from adults far faster than from each other, and you get practice against speed and unpredictability you will meet at any open tournament.

Safety comes first, because you are often bigger and stronger. Never hit at full power: extend your arm early and let the blade bend rather than punching the point in. Avoid crashing into a small fencer, and slow down at close quarters instead of forcing a hit through. Aim at clear target and keep your touches light. The reverse matters too: a strong teenager with a year of training can hit an adult very hard. If someone hits too hard, stop, mention it calmly, and tell the coach if it keeps happening.

To make the bout useful for both of you, agree on a goal before you start. You might only parry and riposte, only attack, fence to the arm, or give them a few touches head start. Fence at the edge of their level rather than winning 5 to 0 or obviously handing them touches; kids notice when you throw a bout. Afterward, one specific piece of praise or a single tip beats a lecture, and if their coach is in the room, leave the coaching to the coach.

Keep things in the open. The SafeSport policies that USA Fencing clubs follow ask adults to keep one-on-one interactions with minors observable and interruptible, which in practice means fencing on the main floor where others can see, not private sessions out of sight, and contact limited to normal fencing and a handshake. To decline, a simple 'not tonight, I'm working with someone' is enough. If a particular kid's behavior bothers you, raise it with the coach rather than handling it yourself.