The "standard" duration for the
Camino Frances (St. Jean Pied de Port to Santiago) is 33 days, plus or minus. This is what the seminal guide by John Brierely proposes. Most ADULT pilgrims aim for this. Remember, your legs as a grown up are much, much longer than any eight-year old.
Each time I have done the Frances, and solo, it has taken between 36 - 38 days. This provided needed rest days at Burgos, Leon and Astorga. I usually tell adults to allow 40 days or so, relax and enjoy the journey. Taking rest days every week or so helps as well.
My other advice is that if you cannot devote enough time to do it fully, enjoying each day and not rushing, break it up into stages or segments. Many, many people do this and take several years to complete the entire route.
If you are time-limited, my recommendation would be to start at Ponferrada. There are good bus and train connections to here from Madrid and elsewhere in Spain.
This is nominally about 220 km east of Santiago and would take 10 days for an adult. For an eight-year old, possibly several days longer. There is a VERY cool Knights Templar castle there that you will want to make time to visit, perhaps the day before you start.
However,
the advice above about booking early is spot on. Reserve early, confirm, then confirm again. If you must change your plans ALWAYS notify the property that you are arriving late or not at all.
When you do you reservation planning, consider that, if you can comfortably walk - say 24 km in 6 hours - the kids MIGHT be able to cover no more than perhaps 18 km in the same time...provided you can regain their attention and focus, and they do not throw a tantrum and refuse to walk further... There are enough cafes at regular intervals that they can be plied with snacks, etc.
You should plan accommodation reservations accordingly. Also, along this stretch, backpack and luggage transport services are commonly available from town to town. So the kids need only carry a school backpack with things they might need during the day. Things like toiletries and extra clothing can go ahead in the mochila transport vehicle. Search above for mochila transport
Camino Frances.
Walking in April on the Frances, you WILL encounter light but wet snow at elevations over 700 metres ASL. So, the kids need to have a layerable fleece, a shell parka, a poncho, a warm beanie and gloves. I recommend microfleece or microfiber for everything but the poncho. You must keep them warm, even if they get wet. On the Frances, snow is possible through the end of May. Hypothermia is a risk if one becomes both cold and wet. Wet and warm is okay. You will dry out.
To answer one of your original questions, no, you are not mad, and this would be a most excellent adventure for the children. You just need to thing and plan ahead accordingly.
Perhaps others, here in the forum who have traveled with younger children (pre-teens) in the spring, could offer additional commentary and advice. Not having done so, I am extrapolating based on my experience and direct observations.
Hope this helps.