What a lovely thread. I recently learned Jake's song along with his intro and am really sad to learn he made up Molly. Regarding the counting, I'd like to add two comments :

As Welshwoman says pump (5) is pronounced pimp, that makes it related to the Yan, Tan... versions that contain pimp (5)

According to what I know as Grimm's Law, p and f are often swapped between languages (as are b and v, g and w) so what struck me there was that the Welsh pump for 5 seems related to not only pimp and pip, but also fip, the German funf and also, after a little hardening of the last consonant, the english five (?)