Children are so eager to play on the piano before they can actually "read the notes." One great way for them to make nice musical sounds and practice playing with curved fingers on fingertips is to compose their own "all black key" song.
First have them choose a theme for their song. (Watching the Stars, Playing Ball, A visit to the zoo, Bike Riding, Princesses, etc.)
Encourage them to play around on the black keys using high and low sounds to represent the theme of their song.
You could ask them to use only "2 black keys groups" or only "3 black key groups" to help them learn to identify the black key groups on the piano.
Playing copycat can also be fun. Play a 2 or 3 note melody on the black keys and then ask them to copy you.