Assuming it's still a little before your end time, you could also say "is there anything else you want me to do before I go?" or "if there's nothing else, I'm heading out"... This would remind them that you're ready to leave, but would still give them a last chance for brief questions/requests.

MB here. Be preemptive.
Give MB a 2 minute recap of the day, and anything of interest. Then say, ok, see you tomorrow. and head out.
Make this the normal routine, so they know what to expect.

Remember, being an employer is new to them. They don't know how the transition out goes. You're the professional nanny. Take the lead