OP, we have a nanny share and have had one for our son since he was 4 months. He’s still in a share at 3 years. I would not be concerned with some of the alarmist others who have posted. I work from home very often and have never seen our nanny prop a bottle or let a newborn cry it out. It isn’t heart wrenching for the other child who gets their own comfy and quiet room to nap in. We’ve had the same nanny over three shares and all her kids just love her.
I do agree, however, that part time is tricky for the nanny and might be difficult for the PT baby to adapt to any schedule. I’d either do a share 100% of the time or try to employ your own nanny for the year.
In terms of pay, we always guarantee hours for our nanny meaning she gets paid like a salaried employee even though she is hourly. She gets the same pay every week regardless if she’s sick/vacation (As long as she has PTO left), we are sick/vacation, mutually agreed holidays. You establish a one child rate for the hours your kid is in the share ($22-$24 sounds like the rate in your area) and then a share rate for the hours the other child is in the share (unknown what this is, ask the nannies you are interviewing but you’d split this hourly rate in half). Keep in mind overtime and how it’ll affect pay. Also keep in mind holiday bonus and yearly raises.
As for sick, we are more lenient in our share than daycare which seem to have very strict policies. Basically for us it is no fever, then fever free for 24 hours, and obviously keeping the kid out at the hint of anything super contagious like stomach bug or skin conditions.