I agree with the above poster. As the saying goes " a bird in hand is worth more than 1000 out there." Say yes you will do it. Even if you end up quitting, you can certainly still help them out those few days, nothing wrong with that. you did say they aren't bad people just job wasn't a good fit.

Be careful, you may not find another job by then.. you don't want to be up set with yourself that you didn't take that job

Keep it simple. Tell them that you cannot send do not give explanation
"No" is sufficient.

Make up something that you have to do.
or say "I don't do overnights until at least 4 months working, to make sure I'm entirely comfortable with the kids, house and emergency situations"

Simply say that you cannot do it that weekend - make up something if you feel you need to (parents are visiting, brother is visiting, promised to help a friend move that weekend, scheduled dental surgery that Saturday, are a bridesmaid in a wedding out of town, etc).

All the best in finding a new position, OP.

Unless you will be REALLY down & out financially, you have to give them a hint at this point.

Because in a sense, you are now "stuck."

You can amicably let them know the areas that you are unhappy with.
Perhaps there is a way that you can all work together to figure out some solution that is best for everyone.
Since jobs are very scarce in your area, this may be the wisest outcome.

However if this option is simply a no-go, then I would decline the overnight (you are not obligated to state a reason.)

Perhaps that may be a clue that you are unhappy and may be looking for another position.

Whatever the outcome, I wish you only the best.

Just answer her with, "No. Sorry." Or "No, sorry, I don't feel comfortable doing that."

So, I'm quitting my current nanny job. I've only been with them for one month, and it's just not a good fit. I'm 100% sure of this decision. I haven't given my NF notice yet, because I haven't found a new position yet. I live in a rural area so it's a bit difficult to find a job, and I don't have any leads yet. I'm working on it though. I do plan to give them 3 weeks notice because I know that they'd be in a huge bind if I gave little to no notice. Nannies are hard to come by in my area and I don't want them to have to rush and bend over backward looking for care. They aren't terrible people, we just aren't a good fit together.

However, MB just texted me asking if I could do an overnight with the kids in late August while she and DB go on a trip. It would be three days and two nights. I fully intend on having a new job by then, which obviously she doesn't know, so I don't want to commit to doing this overnight for her and then leave her in a bind later when I quit and she has no one to work the overnight. I also don't want to drop the bomb now that I won't be with them at that time, for obvious reasons. I don't feel comfortable giving notice if I don't have something else set up already.

How should I handle this??? Tell her I'll check my schedule and get back to her? Not sure how long I can keep that up, as it may be weeks before I find a new position and give my notice. At the interview, I told her I was fine with occasional overnights so I can't use "I don't do overnights" as an excuse.