Underage magic

[ladyramkin2000]

No: HPFGUIDX 88592

In Chapter Four (The Keeper of the Keys) in PS, Petunia refers to her 
sister as "coming home every holiday with her pockets full of frog-
spawn, turning teacups into rats".  Is this just Petunia being 
spiteful, or did Lily really practice underage magic? And why on 
earth should anyone walk around with their pockets full of frog-spawn?
Or is this just Petunia again, just saying whatever comes into her 
head to get years of frustration off her chest. 

Sylvia (the teacups into rats mention seems to ring a bell, but I 
can't place it.  Or am I getting confusedwith the films?)