Well, like getting yourself to the toilet or not it is not one of those things you've either 'got' or 'not got'. It depends on circumstances (very absorbed in what you are doing - might not get to toilet), state of health and well-being (sad or poorly - might not get to toilet), amount of support (noone to see me wriggling - I didn't get to the toilet). You might have it one day (Hurray at last little Fred is toilet-trained) and not have it the next (Oh no, he's regressed). Of course you do end up, at last, getting it. So, you can play a game that demonstrates conservation of number and some children will know it (probably already got it), with others it will click (within the context of the game but maybe not for ever), while others will be introduced to it (it might click next time),and some may not even notice or follow it at all, but they will have fun and do a bit of counting. They will all be somewhere on the continuum. Don't set too much store by Piaget. Read, 'Children's Minds' by Margaret Donaldson.