It also depends on their age and what they're ASKING. Don't give them more info than what they want. It could be more confusing and overwhelming.
SG and AB's kids know that Mummy and Daddy love me very much and wanted me to be a part of their family. The kids are 2, 7 & 9. They have seen little kisses, cuddles, pats on the butt etc, and seem to be doing fine with the affection. When THEY ask more questions, it will be addressed then.
"Thou art to me a delicious torment." - Ralph Waldo Emerson