Is anyone here familiar with the play Kindertransport? Does anyone know where I can get the music to the two songs in the show? Thank you! 

My wife is from Germany.  She was familiar with the song mentioned above in as much as her parents used to sing it to her while bouncing her on their knee as an infant.  The song was sung traditionally to make the infant laugh as they were dropped (and lovingly caught) from a parent's lap.  She did suggest a few changes to the lines above.

Hoppe, hoppe Reiter (hop, hop rider)

Wenn er faellt dann schreit er. (when he falls he cries)

Faellt er in den Graben (if he falls in a ditch)

fressen ihn die Raben (the crows will eat him)

Macht der Reiter plumps (the rider goes "BUMP")

faellt er in den Sumpf (and falls into the swamp)

Cute eh?  I don't know if this is helpful, nor am I remotely familiar with any stage production in which the song is sung.  To find music for it, you might look for a book of traditional German children songs.