When I have had to do an observed task like this for an interview, it has always been a very simple activity - read a story, a bit of handwriting or practicing a few phonics sounds. I expect they're looking to see how you relate to children, not whether or not you can turn something simple into something whizzy and amazing. As an experienced TA with children of your own, I would totally expect you to sail through it - so I really wouldn't be nervous about that part (save that for the formal interview bit!). They're not trying to trip you up, they just want to see you with children. And I don't agree with the first poster, although I could be wrong, as when I have done observations at interview it's been a fair fight between all the candidates - so you could definitely be in with a good chance. Good luck!!!