Hi
@AndaPronto - lovely to see how you and children are progressing. Please keep us informed as you go along (start a new thread with the "live from the camino" at the beginning of the thread title).
As to your questions, there should be shops open in SJPDP past 20:00 at this time of the year (although they do not stay open as late in France as they do in Spain) and I think the albergue at Orisson will provide a lunch pack for the following day if you let them know in time. Some concentrated trail mix is an easy thing to carry; if you are concerned buy some before you get on the train for SJPDP.
You are right, there is nothing, absolutely nothing, between Orisson and Roncesvalles. Unless you are lucky; sometimes there is a mobile drinks van parked near the Vierge de Orisson.
More important than food, is water. Good that you have camelbacks - although I'm not usually a fan in this case it is useful as you will need plenty. There is a water fountain just at the border crossing into Spain, over the top, but it is a long way til then and I'm not sure if it is always running.
I walked the Route Napoleon one year in June, with my daughter, and it was very, very hot. Until we got to the descent side of the route there was virtually no shade. The Pyrenean vultures were circling overhead in a large flock and we thought we were being sized up as their next meal, when we expired from heat exhaustion.... Still, we made it and the views were spectacular.
I would be taking cheap umbrellas for everyone - get them from a $2 shop and it does not matter is they get thrown away later (although I love my umbrella for rain as well).
The third time I walked was also hot, clear and sunny, but we were blessed with a cool wind. Lovely.
As to your question about alternative transport - there is a road that goes a long way up the Route Napoleon - quite a bit past Orisson. So there is no need to decide now to skip Orisson - if you get there and really think the next day is too hard you could call a taxi from there. It does not have to go back to SJPDP as there is a road across to the Valcarlos crossing and from there to Roncesvalles.
I agree. Although not usually an early riser on Camino, in this heat an early start from Orisson is the way to go. Don't wait for breakfast.