I want my content types to have more than one child content type in drop down lists.

Background to my problem:
I’ve got different content types that have different child content types.
Depending on what child content type you choose in a drop down list, different fields connected to the child content type show up. No problem so far.
For the parent drop down list and the child drop down list I have used the book module. But that only alows you to have one child content type. Which means that the parent drop down only can have one ”child drop down list”. I want there to be at least two or three ”child drop down lists”. So my problem is that I want my parent to have several children or if you like, the child to have siblings (or maybe you call them cousins in this case...?).

Either way I want to be able to set a Condition to each child content type as explained below.

This is just something I made up as an example:

In the first drop down I choose Vehicle and then in the second drop down (with the child content types to vehicle) I choose Boat. Now different fields connected to Boat show up which they are supposed to, all is good this far. The thing is that I now also want a new drop down where you can choose the condition of the Boat to show up (new, old etc). Depending on which type of condition I choose I want different fields to show up. So for example if I set the Boat condition to be old I want the fields for old to show up and if i set the Boat to be new I want other fields to show up and so on...
Does any one know a good solution to this? Thank you for helping!

BR
akgr1