While driving from Alaska to Virginia this summer, we stopped to see some friends and family in Utah. A spontaneous family session was booked while we were in Utah, and we pulled it together in a matter of hours. We went across the street to a historic village, and with *some* bribery, several setups, and a bit of young child attitude, we had the looks we were going for.
Although it is often preferable to schedule in advance to give the family time to get outfits and other details ready, all of that is really secondary. The most important is to be there - and sometimes, spontaneous is the best way to show up.