Both are so special...hands down I would not miss Grannon, though I know you are going with children and Tosantos had some very special traditions that I think they would not forget AND would not experience anywhere else.
I personally think, especially with kiddos that you can do both...I did
It was my shortest walk of the trip and might not be justified but I think 100% of us in Tostantos that night had been in Grannon the night before.
I enjoyed the street of Grannon, I stayed in the parochial there BUT was lucky to chat with the owner of the auberge up the street who honestly could have been the welcoming mayor of this little town. I would not trade my Grannon night for anything.
Tostantos had one simple , VERY simple bar...it was probably my most lazy day in that really there was nothing around it (which was fine with me) BUT it did come with a tour of a church on the cliff, a precious meal around ONE table and a meeting up in the attic like structure of sharing stories and wishes etc...
Grannon too had the awesome meal and we all helped! Grannon was overflow when I was there and just kept welcoming the masses (loved this)...we almost sat in eachothers laps as we ate. We too got to go to a church service there AND they opened up a little section filled with religious art and stuff. We sang for our meal , I don't want to share too much as not knowing is fun
I might not have helped you much here BUT have done the full Frances and pretty much planning, like you, to do it with kiddos SO have revisited my trip in mind with that as consideration.
Buen Camino!
Neve