"to wimp out"
to not do something because of cowardice or lack of conviction
abandon, accede, acquiesce, balk, cancel, cave, chicken, coward, cowardice, demur, fear, fearfulness, give, gutless, recant, recoil, renege, resign, retreat, submit, surrender, timid, withdraw, yellow, yield
The little girl is refusing to cross the swinging bridge because she is too afraid. She is wimping out