"Since she was a child" might not work in all circumstances, as it also means "because she was a child".
The Queen fell in love with Corgis as a child.
The Queen's love of Corgis began in childhood.
The Queen loved Corgis all her life, since her childhood.
Use "had loved" when you are speaking about a specific time in the past, then describe a period before that.
The Queen was given Roger in 1995. She had loved Corgis since her childhood.
In 2010, the Queen stopped buying any more Corgis even though she had loved them since her childhood.