They play the audio CD of Mustard, Custard ... more than any of their music CD's- and that includes the Frozen soundtrack 
In particular my 3yo likes 'Don't' and 'don't throw fruit at the computer' is driving me crazy so I need a break.
What else would they enjoy? 6yo dd1 likes following along with the book so a combination of book and CD would be perfect.
Preferably poetry - something funny?