I try to set "bubble" to titles only, not for all children, is that possible?
Different solutions depending on version of FP do use: 1.1.3 or 1.2.
In general, choose "Combined" in stead of "Bubble". Then Bubble appears if
node is folded.
A different way is to use automatic level styles and give (only) the highest
level(s) a bubble.
In FP 1.2 beta also conditional styles may be used. See the
tutorial/documentation for FP 1.2. on formatting and styles