Numbers of children welcoming first appearance of grandchild (5)
I believe the answer is:
songs
'numbers' is the definition.
(musical numbers)
'children welcoming first appearance of grandchild' is the wordplay.
'children' becomes 'sons' (son is a kind of child).
'welcoming' indicates putting letters inside (in sense of 'inviting in').
'first appearance of' suggests taking the first letters.
The first letter of 'grandchild' is 'g'.
'sons' going around 'g' is 'SONGS'.
'of' is the link.
(Other definitions for songs that I've seen before include "Arias" , "Airs" , "Words with music" , "Melodies with words" , "Verse" .)