I used the 400mm f/5.6 for many years before buying the 100-400 mark ii. If you're looking for a prime lens that isn't too heavy to carry around, I'd recommend the 400 F/5.6 if you can find a used one. I can honestly say that I was always pleased with the lens, except for the fact that it wasn't a zoom lens, it's not very practical when a deer is less than 10 meters away from the car window unless you want to decide between capturing the antlers or the face, or half and half. For birding it's a great lens, the fact that there's no IS built into the lens wasn't a big deal for me, it just means it's a lot lighter to carry.
Now having said that, if it was me spending your children's inheritance, I'd definitely buy all three. Maybe put a clause in the will so the kids inherit the camera gear, it'd still be part of the inheritance for the kids and you'd get new camera gear, sounds like a win-win to me. ?