Between my nanny and I, we used to take my younger one along to pick ups every day for a similar round trip time for a couple of years - and to be honest they do get used to it and it was never a problem - it was normal to them. And there are certainly always a number of younger siblings at the school gates at drop off and pick up. My nanny used to take the buggy on the bus, but you might want to insure the nanny on your car if that is easier and quicker than public transport.
Certainly lift sharing is great, especially if you can reciprocate in some way and the other party are getting something back for helping you out and cuts down on the amount of school runs you are doing - but yes, difficult to arrange in advance unless you know parents before you start. Some schools promote car sharing etc, so you could ask the school if they do that.