not sure that Truly Scrumptious is actually sung by children
Yes, it is. It's sung by Jeremy and Jemima to Truly, who then joins in. (There's also a reprise, sung by their father Caractacus to Truly - both disguised as dolls.)
ORIGINAL LYRICS:
Jemima
Toot sweets sound like what they are
Jemima
So do lollys in a lollypop jar
Both
Gingerbread men have a gingerbread sound, we've found
Jemima
Sugar plum cinnamon and lemon tart
Tell you what they are right from the start
Both
And your name does the same for you
By coincidence, Truly Scrumptious
You're truly truly scrumptious
Scrumptious as a cherry peach parfait
When you're near us
It's so delicious
Honest, Truly, you're the answer to our wishes
Truly Scrumptious
Though we may seem presumptuous
Never, never, ever go away
Our hearts beat so unruly
Because we love you truly
Honest Truly, we do
Truly
Truly scrumptious
You two are truly scrumptious
Scrumptious as the breeze across the bay
When you're smiling
It's so delicious
So beguiling
You're the answer to my wishes
Truly scrumptious
You two are truly scrumptious
And I shan't forget this lovely day
My heart beats so unruly
I also love you truly
My heart beats so unruly
I also love you truly
All
Honest, Truly
Truly
I do