More importantly than what you tell your guests, I’d have a real heart-to-heart with your cleaners. It could be that back-to-backs are too hard on them, and more cleaners are needed to get it cleaned on time. Or you may decide with them not to schedule back-to-backs at all (not a bad idea during these COVID times).
Communication is also important - if the housecleaners communicate with you that they might run late, you can message the guest and find out what time they are arriving, and ask them for an extra hour or 2 if they were planning to arrive right at check-in time. This is preferable to having the guests show up at an unclean or half-cleaned place, and they may have not been planning to show up right at check-in time anyway.
IMO, a clean place is paramount to a good stay. It is the very first impression your guests experience when they walk through the door. Therefore, I would definitely offer compensation, or something fun like a discount off a good restaurant, etc. if they do indeed arrive at your place and it has not been cleaned, or is late getting cleaned. Your idea of offering a late check-out is a good one, too.