"went up the hill" is the predicate, as it describes the action that Jack and Jill are doing.
Jack and Jill went up the hill to fetch a pail of water. They went there because they needed water for their household chores.
Yes, they went up the hill to fetch a pail of water.
jack and jill should avoid the hill top. because when jack and jill went up the hill, jack fell down.
jack and Jill went over the hill
two subjects = Jack and Jill - Jack and Jill went up the hill.
Yes.
Jack and Jill went up the hill to fetch a pail of water Jack fell down and broke his crown and Jill came wandering after.
Jack and Jill ( . . . Went up the hill . . .)
after that they rolled down the hill
Jack and Jill went up the hill. Jack or Jill fell down again.
Jack & Jill