We had one for DD since she was about 18mths. Didn't really get it at that age but we just started saying it "Say goodnight sun, look the moon is up, the stars are out! Now you stay in bed and stay quiet until the sun comes up!" and eventually she got it. DS got one for his second birthday (so just recently) and he already gets it, thanks to saying goodnight to DD's sun when reading books etc with her and DH in DD's room. He wasn't quite getting the morning thing though until just the last few weeks - I hear him chattering away around 6am (when DH leaves, I think DS is a light sleeper like mummy) then he comes skitttering down the hall to our room and announces "Mummy, the sun's up! Wake up time!"

Definitely recommend one.