For kids, train two dragons (4-4)
I believe the answer is:
puff-puff
'for kids train' is the definition.
I can't tell whether this defines the answer.
'two dragons' is the wordplay.
'two' indicates the doubling of some letters.
'dragons' becomes 'puff' (I am not sure about this - if you are sure you should believe this answer much more).
'puff' doubled is 'PUFF-PUFF'.
Can you help me to learn more?
(Other definitions for puff-puff that I've seen before include "Child's train" , "Choo-choo" , "Steam locomotive" .)