Use repetition, clear melodic patterns, simple vocabulary and grammar with some ridiculous concepts or or words. And don't forget to add something naughty!
All Together Now
I Am The Walrus
Barenaked Ladies, Belle & Sebastian, St Etienne, They Might Be Giants have all produced children's albums and Chris Ballew (Presidents Of The USA) leads a child-friendly side project called Casper Babypants.
Ticket 23: Write Fewer Lyrics
Ticket 42: Have A Completely Unique Concept For The Song
See the full list of songwriting tips here - Tickets To Write