There are lots of ways of playing this shaker but most very young children will just shake it!
You can also do more technical things like holding the cascas through your fingers so your palm is facing upwards like half a 'prayer-hand'. Put one of the cascas shakers so it is trapped on the inside of your hand and the other dangling down. Now just swing the cascas forward and bwackward. It should hit the othe cascas so that it makes a hitting sound as well as a shaking sound.
Of course babies won't be able to do this yet but you could try it and it'd be very interesting for a baby to see and hear.