Hmmmm.... I've never really read anything about toddlers being more mixed-handedness/cross-dominent and then moving from there as they age to a single handedness. I'd be interested in where you learned that from.

The big problem is experts still don't even agree on the reasons behind handedness (though the popular theory does revolve around the whole left brain/right brain concept, but others suggest that left handers are such due to slight brain damage during birth).