Interesting post but I do think you are trying to control too much ... sure, it is fine to go without planning or training, whatever other people may say - the whole trick is to take it easy and listen to your body and build up distance - should you even want to! - after a week or so - if you look at the refugee crisis you will see that quite sedentary people have trekked hundreds, even thousands, of miles with children and carrying babies and with no 'trekking' kit.... and it is only the Camino you are going on!
Certainly St Jean to Roncesvalles is a long day but it isn't Everest .. the road route, for instance, is graded so that large trucks and small cars with caravans can drive it - hardly sherpa stuff ...... and! you can split it into two days of you wish, so I would say just go for it! and Enjoy .. what marvellous family memories you will have.
what do you mean by pre-teens? Really young or close to teen? They tend to be inherently fit you know, and there is something about the weight to muscle ratio that allows them great endurance without harm, as long as they get to sleep lots!!
I used to organise one day charity bike rides, large scale events of thousands of cyclists and we always had a few 7, 8, and 9-on children on their heavy little bikes (along with their parents!!) - they seemed to have absolutely no problem whatsoever cycling the 50 miles - quite extra-ordinary really.
As for your own problems, well, only you can decide how you are and what you can do ... you don't have to walk far each day you know, and if you start really early you have the whole day to stroll as far - or not- as you want .. but all of you, pack light,
no, really, pack light - and have your children carrying their own things - this will force them to leave all the extras behind - I never carried my children's things on walks .. I'm not a donkey and thought they needed to know early that if they thought they wanted something then they had the responsibility to carry it and look after it. That worked out very well.
My first Camino and preparation? A week or so before I walked with my loaded pack a return loop of 11 kms just to see if it was comfortable, bought new boots and wore them on that 11kms .. then about ten days later set off by train to my starting point (Moissac, in France) .. I seem to have survived it! (I was 57 then)
Let us know what you decide - ignore anything you hear that has to do with fear and being too careful - too much Health and Safety out there nowadays ... go out there and just stroll along and enjoy yourselves
Buen Camino!!
p.s. Of course you are crazy - anyone who wants to leave home and walk the Camino is crazy - welcome to the club!