DD2 is 4.5 - she can say the 'th' sound but will not use it in conversation, it's always pronounced 'd'.
For example, "Look, this bit comes up and that bit comes off... someone did that, didn't they?" Becomes:
"Look, dis bit comes up and dat comes off... someone did that, didn't dey?"
To encourage her, I'm asking her to repeat each word correctly.
Nothing was said at Nursery, she was assessed as 'Emerging' at speaking for 40-60 months.