Hi, kiwi-family,
You certainly have a lot of options, and I don't know how you'll choose! If you want to walk St. Jean to Leon, you could walk the 120 kms north to Oviedo on the Salvador and from there to Santiago on the Camino primitivo. That combination is really terrific, but I don't know if the elevation gains would be too strenuous for the littlest ones. As albergues are being opened with some frequency, you can find ways to do it with shorter distances, but there is no way to avoid the elevation gain. If the elevation were too great for some, it's a short bus or train ride to Oviedo, and you could walk from there to Santiago on the Primitivo. You should look at that section of the forum, because there are lots of good posts there on stages and accommodations. Tia Valeria and Tio Tel have walked the Primitivo in shorter stages and have offered a lot of help to those who want to walk this beautiful route without the long distances. The primitivo is an absolutely wonderful camino, perhaps my favorite. It has incredible scenery, great albergues, and just enough pilgrims (neither too many nor too few). It also goes through the lovely little city of Lugo with its Roman walls in tact.
Another option would be the Via de la Plata from Sevilla, or if that's too long a distance, you could start in either Merida or Caceres. The Vdlp in springtime is a wildflower wonderland, it has lots of flat walking up till Zamora, and from there you hit some elevation gain. It is also one of my favorites because the cities of Sevilla, Merida, Caceres, Salamanca, Zamora, and Ourense are all wonderful places to spend some time, just loaded with history from the Moorish rule onward. There are a couple of places with long stages but creative work-arounds are possible.
I have many wonderful memories of stunning seacost scenery on the norte. It hugs the coast (or stays more or less close to the coast) until Ribadeo and then moves southward through Galicia. Lots of pretty tourist towns on this route, since it's on the coast. And the cities of San Sebastián, Santander, and Bilbao are all great. Lots of people here have walked it more recently than I and can offer more recent advice.
As you saw from your question on another post, the LePuy-St. Jean part is more expensive for lodging and food. But beautiful as well, so I just don't know how you will choose!
There are other possibilities as well, but the less travelled caminos are likely to have little if any infrastructure, meaning private pensiones most of the time. They tend to be on the low side for an individual, in the 15-20 euro range, but multiplied by your numbers it would really be quite expensive.
I'm sure others will have other suggestions to add, so hang on! Buen camino, Laurie