Is there any pattern/algorithm to know number of some property in dynamic tree?

I have a dynamic tree - nodes can be added/removed anytime. Every node:

• can have N children;
• has array of foo;
• can add/remove foo to/from to his own foo array.

And I always need to know how many total foo in tree. Is there any pattern or algorithm for such problem, taking into consideration there is always only one thread (no multithreading)?