My DD is four, she has had speech delay but is fine now, well - i think she is and the school haven't raised any issues about it. She does struggle with some words and my mum struggles to understand her sometimes.
I would never dream of correcting her on words she is struggling with, although i do sometimes just say the word properly but she has developed a habit of dropping her Ts and it drives me nuts - her dad is from south east london so is the king of Bu..er wa..er etc. I do ask him to try not to do this in front of DD but its the way he always speaks. But in light of her speech delay should i just let her talk like it - i honestly think its what she has learnt rather than her speech problem and i correct it. She CAN say her Ts in words and as i said, if she had problems prounouncing it i woudlnt correct - she struggles with G sounds and C sounds sometimes, but i just ignore it.