Some obvious sugestions:

If childhood were music it would be
Mozart:  Ein Madchen oder Weibchen- aria(Papageno) from The Magic Flute
(and there is much more enchanting innocent music from this opera).
Brahms:  Berceuse
Liszt:  Consolation #2
Schumann:  Traumerei from Kinderszenen
Chopin:  Prelude op.28,#6 (if I'm not wrong this is the simplest, loveliest
one)
Beethoven:  Alla Danza Tedesca from one of the last string quartets
Bach:  Prelude #1 in C major from the Well-Tempered (book I)

best regards
Marcelo