Once he's outside it's hard to get him to go back inside. He keeps on playing.
Once we go outside it's hard to go back inside. He just wants to keep on playing.
「一旦」は “once” と言います。解答1の “it’s hard to get him to go back inside” は「彼を室内に帰らせるのが難しい」の意味ですが、解答2は息子さんの事を言っていなくて “it’s hard to go back inside” 「帰るのが難しい」になっています。解答1の “He keeps on playing.” は「彼がずっと遊び続けている」事で、解答2の “He just wants to keep on playing.” が「彼がただずっと遊び続けたい」です。