You're not short of time. You're short of a way to use the twenty-five minutes that turn up. The window opens without warning: they've fallen asleep, they've gone out, grandma's here. You can join the next free slot right away, without waiting for a time booked in advance.
People with young children don't work in long blocks: they work in the gaps, ten or thirty minutes between a nap and an outing. The problem isn't that the gaps are few: it's that they often go by deciding what to do with them.
A twenty-five minute session fits inside that window with nothing left over. And the total at the end of the week no longer comes from memory: it comes from the log.
Not always at that exact moment: if nobody is there yet, for the first 5 minutes we look for someone while you work. If nobody arrives, you choose: carry on with a virtual companion, and if you stay until the end the session counts in full, look for a person in the next quarter hour, or leave. Booking a fixed time in advance is still the option most likely to find someone already there.
Yes, you can leave the session at any time: the minutes you've already worked stay yours in the log.
No: there's a slot every quarter hour, all day, and you can join even a few minutes before.