cd current_tree
bzr export ../child_tree
cd ../child_tree
bzr add-from ../current_tree .
bzr commit -m "Creating new child with no history"
cd ../current_tree
cat ../child_tree/.bzr/checkout/last-revision \ 
   > .bzr/checkout/pending_merges
bzr commit -m "null merge from child"

# At this point current_tree should be able to merge new changes from the child
# but the child should have no real history