Plurals, Possesives, and Proper Nouns ending with 'S' [duplicate]
The last version would only be correct if the children were the offspring of someone called Jackie Wallses. Your response made sense to me.
Otherwise, either Walls' or Walls's are both in use. Wikipedia has a nice section with lots of other references, and suggests that the first is archaic. I believe it's also used a lot in poetry.