Childhood malady (7)
'childhood malady' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for measles that I've seen before include "Children's disease with red spots" , "Morbilli is technical term for this illness" , "I appear as 8 [RASH]" , "Rash-causing disease" , "Children's viral disease" .)
(Other definitions for chickenpox that I've seen before include "Disease" , "Youngsters spotted with it" .)