The important part is not that the opponent is a kid, the important part is that they're likely to be inexperienced.

The goal of this social event is likely for all participants to have a good time. Bob is one of the staffers, so he has been tasked with making the attendants have a good time.

If the kid just wants to "push sticks around", as my grandfather used to call it, they'll get bored no matter what, so there's no way to make the kid have a good time, except by letting them play with the pieces.
Once the kid gets bored, there's an opportunity to teach them chess. If they're not interested in that, there's nothing else to do except waiting for the parents.

But should the kid want to play, and let's assume that is the case, Bob should play as he would against any inexperienced player, and adjust his playing level according to the perceived level of his opponent. Warn his opponent of really bad moves and let them take the move back. Explain. Think out loud. Ask his opponent why they make a specific move. Go through the options together.

To keep the game interesting for Bob himself, he could try out some unconventional moves and see where they lead. An experienced player would punish those, but an inexperienced player may not now how, so this is an opportunity for Bob to explore a bit.

The important part is not that the opponent is a kid, the important part is that they're likely to be inexperienced.

The goal of this social event is likely for all participants to have a good time. Bob is one of the staffers, so he has been tasked with making the attendants have a good time.

If the kid just wants to "push sticks around", as my grandfather used to call it, they'll get bored no matter what, so there's no way to make the kid have a good time, except by letting them play with the pieces.
Once the kid gets bored, there's an opportunity to teach them chess. If they're not interested in that, there's nothing else to do except waiting for the parents.

But should the kid want to play, and let's assume that is the case, Bob should play as he would against any inexperienced player, and adjust his playing level according to the perceived level of his opponent. Warn his opponent of really bad moves and let them take the move back. Explain. Think out loud. Ask his opponent why they make a specific move. Go through the options together.

To keep the game interesting for Bob himself, he could try out some unconventional moves and see where they lead. An experienced player would punish those, but an inexperienced player may not know how, so this is an opportunity for Bob to explore a bit.