Yes, you can achieve it by find_elements_by_css_selector("*") or find_elements_by_xpath(".//*").
However, this doesn't sound like a valid use case to find all children of an element. It is an expensive operation to get all direct/indirect children. Please further explain what you are trying to do. There should be a better way.
from selenium import webdriver
driver = webdriver.Firefox()
driver.get("http://www.stackoverflow.com")
header = driver.find_element_by_id("header")
# start from your target element, here for example, "header"
all_children_by_css = header.find_elements_by_css_selector("*")
all_children_by_xpath = header.find_elements_by_xpath(".//*")
print 'len(all_children_by_css): ' + str(len(all_children_by_css))
print 'len(all_children_by_xpath): ' + str(len(all_children_by_xpath))
In 2022, with selenium==4.2.0, @Richard's answer will need to be rewritten as:
from selenium.webdriver.common.by import By
parentElement = driver.find_element(By.CLASS_NAME,"bar")
elementList = parentElement.find_elements(By.TAG_NAME,"li")