Where is thumbkin, where is thumbkin? (Hands behind back.)
Here I am! Here I am! (Bring right hand out from behind back with thumb extended ["thumbs up"] on first phrase, then produce left on second phrase.)
How are you today, sir? (Move right thumb as if talking.)
I am fine, thank you! (Move left thumb as if responding.)
Happy day! Happy day! (Clap.)

Repeat for other fingers (pointer, tall man, ring man, pinkie).

Suggestion: After students become familiar with song, substitute children's names and use it for class opening. Alternate these words:

Where is [student], where is [student]? (Teacher sings.)
Here I am, Here I am! (Student responds, waving.)
How are you today [sir or ma'am]? (Class sings.)
I am [fine, happy, sad, tired, hot, cold, etc.], thank you. (Student responds alone.)
[student] is [feeling], [student] is [feeling]! (Class responds.)