Olá Marcelo! Muito legal ver seu empenho. Minhas sugestões:
Once upon a time a little ant was looking for food and / when it ran into a challenge / was faced with a challenge. It had to cross the river in order to get (to) the food, but it didn't know how to do that.
While it was thinking how to solve it, an elephant came and said at once: "Hello Mr. ant, why are you so thoughtful? The ant replied:" I have to cross this river, but I don't know how. So the elephant said: "I can help you!", and the little ant squeaked / answered:
"Really? How great!". But the elephant said: "I want something for it! The ant: "I can give you anything! Then the ant got up on the elephant's shoulder and they both crossed the river. When he / it was on the other side / after reaching the other side, the little ant, who was sneaking away, said:
Thank you very much Mr. Elephant! (And) the elephant said: Litlleeee antttttttt! What about our deal??
Lembrando que "across" é o advérbio ou preposição ("to swim / wade / walk across the river"). O verbo seria "to cross".
Quanto a "twitter": eu penso que "twitter" é mais comum em referência a pássaros. "Birds twittering in the distance". "Squeak" seria uma opção mais genérica e abrangente, descrevendo o "gritinho" de uma formiga.