It is OK to not look at music and just play a song as you know it. It might take a few tries to figure it out, but it is one way to learn where the notes are. I don't know if Aussie's know Mary Had a Little Lamb, or Row, Row, Row Your Boat, but songs you learned to sing when you were little are good to try. It is very good for your brain to do this. Reading music takes a while to learn, but your ears know what they are hearing now.
The world-famous Suzuki string method begins with learning Twinkle, Twinkle, Little Star by ear only. And by very young children too. Because they can play a song they know so quickly, they gain confidence and it becomes fun for them, making them eager to learn more.
Even older kids can benefit from this approach.