Just that - the phrase 'Oh my God...'
I work as a TA with kids aged 8-10 and so many of them say this whenever they are frustrated, stroppy or feeling argumentative. I'm not religious but I certainly consider it an inappropriate phrase for children to use.
I had a chat with a couple of the girls today after one of them said it. Neither of them had any concept that this was anything other than normal, a it's a phrase used by all the family. I suggested that they might like to think of other things to say that won't sound so offensive or rude.
I fully expect that a couple of parents will complain about today's conversation if their girls repeat it.
AIBU?