The kiddies have love in their heart, darling (5)
I believe the answer is:
toots
'their heart darling' is the definition.
I can't tell whether this definition defines the answer.
'the kiddies have love' is the wordplay.
'the kiddies' becomes 'tots' (I've seen this in another clue).
'have' indicates putting letters inside.
'love' becomes 'o' (love means zero in tennis).
'tots' enclosing 'o' is 'TOOTS'.
'in' is the link.
Can you help me to learn more?
(Other definitions for toots that I've seen before include "sounds horny" , "Horn sounds - honey" , "gentle Dickensian" , "ducks" , "Honks horn" .)