I think the tricky thing is that they are going to encounter fractions in everyday language before they are at the stage of discussing different fractions as being equivalent. I've got a 3 year old and 5 year old. The 3 year old uses 'half' whenever something is divided in two, regardless of the size of the pieces, even if I correct him. The 5 year old on the other hand is capable of doing addition/subtraction with halves, quarters etc. but hasn't encountered that at school yet and hasn't encountered fractions like 2/4 because people never refer to that in real life. By the time they learn properly about fractions in school, he's already going to have had a lot of experience of using them in a non-formal setting which is inevitably linked to concrete concepts. On the other hand, he has got a mother with a PhD in maths, so I suspect he might get introduced to the concept of equivalence classes rather earlier than most kids :-)
(By the way if anybody out there does have children that age, I recommend the game Fraction Formula which has tubes in which you put cylinders in to representing different fractions).
(By the way if anybody out there does have children that age, I recommend the game Fraction Formula which has tubes in which you put cylinders in to representing different fractions).