Not sure how Yakult would be for that age - you could call the company and ask?
I use a baby biotic - its one that you need to get from a naturaopath but it's awesome. it isnt cheap though.
i am on a mission to boost the immunity of my boys (esp DS2 at the moment) so I also have another immune booster (basically zinc vit c and vit b) - i mix them both up with some water and give it as a 'magic potion'. they are loving it and i think it is helping. i'll keep it up for winter - we have all been so sick and i want to avoid as much as poss.
i have also used brauers recovery tonic - that is pretty good (different stuff) and would be ok at her age (as well as the probiotics etc)