English word stress

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I'm a mother tongue English grandmother living in Italy. I have spoken to my 2 year old granddaughter (as has her father, my son) solely in English since her birth just over two years ago. She understands everything I say but prefers, of course, to speak in Italian but will translate when I ask her to. My question is: if HER English is MY English, why on earth does she automatically change the word stress? E.g. she'll say holiDAY rather than HOliday, shaDOW rather than SHAdow etc. I'm completely at a loss as to why she should do this as she should just be copying, shouldn't she? I'd love to know the reason if anyone can help. Thanks!