I suggest that you get three models that all take the same power pod (so either three normal swappables or three mini swappables), and get one radio and receiver.
I know that you envision your kids all flying together at once, but that's not going to be your reality at first. What's really going to happen is that you are going to have them take turns as you guide them. There is only one of you and so for now you only need one radio. And since a receiver can easily be a permanent fixture to a power pod (along with esc and motor), then you really only need one power pod as well.
As your kids graduate to the point where you feel you can step back and let one fly by himself without you, then get that one his own radio and power pod, in the meantime, you can mitigate your initial investment by having one radio, receiver, esc and motor in a power pod that you can swap into different models. I would get each kid their own battery, though, so no waiting to fly after their sibling.
Also, if you get a speed-built kit, make sure to make some spare parts using the kit as a template.