Add force to Child while it's parent rotate around something.....

hello, i have an object that have one child,this object rotate around a tree,when i move the child by typing "tranform.position=vector3.lerp" inside onupdate,everything OK,after moving , the object and it's child still rotate around a tree with 100% same locus.

however,when i move the child by typing "addforce" or "addrelativeforce" ,the child move ,but the locus of "rotate around tree" had changed? the parent and the child still rotate around the tree but they a in difference locus....

the parent is not a rigid body,but the child is.

it look like that after addforce,the parent still don't know the child position changed....is there a way to renew child and parent relative position?