I have 2 CMs at the moment, one of them only childminds during the holidays & 12 weeks in the summer; it's terrific! Means my regular CM can take hols & I don't have to worry about scrambling for a sub. Regular CM is solid-booked, hard to get places with her, but holiday CM can take us at short notice, juggle days, do extra here and there. She's flexible with me & I'm flexible with her, I work flexitime, anyway.
That said, we are her only client, which maybe says the demand for this flexibility isn't high. I don't know why not. I think it suits the holiday CM that she isn't over-subscribed because she gets some holiday days, too. She also takes both my under 4yo children, too, as opposed to just one child which twiglett suggests, but only 1.5 days/week usually, so plenty of time with just her own 4yo & 9yo sons. The kids just all play together & have a good time.
If you extended to taking 2 children, Twiglett, you'd be that much more popular, because getting occasional CM spaces for 2 siblings is jolly difficult. I have even asked my regular CM to let me know when the 2 siblings she currently minds go on holidays; I would be only too happy to give her mine to look after on those days, too; but she never remembers.