In the Russian language, stress cannot be predicted by a rule.
Russian stress is variable and mobile. It can fall on any syllable and, in addition, shifts within a single word when gender, number, or case changes, leading to pairs like pónyal and ponyalá. Therefore, each form has to be memorized separately, and the best way to do this is with an ear trained on spoken language.
How to help a child remember the stress in difficult words?
Memorizing from a list hardly sticks. Reciting aloud works better: say the word in a short phrase where it sounds natural, and have the child repeat after you. Songs and rhymes are a great help; the rhythm in them simply won't let you put the stress in the wrong place. And difficult spots in a children's book can be marked with a pencil above the needed letter so the child doesn't stumble when reading.
Is it true that some words have two acceptable stresses?
Yes, but there are few such words. Dictionaries mark equally valid options specially, and then both are correct, as in the pair "bárzha" and "barzhá". Much more often, you'll see another marking, where the second option is recognized as colloquial or professional, like "kómpas" for sailors and "dóbycha" for miners. It's better not to use such forms in ordinary speech.
