DD (2.7yo) has a really good vocabulary, but has trouble pronouncing certain letters. For example, F and S become D (so she'll say 'dish' for 'fish', and 'dun' instead of 'sun'). It only seems to be letters at the beginning of words that cause a problem; word with an S or S sound in the middle (like dance) are fine.
Is this normal for her age? Any advice appreciated.