My religious beliefs came in handy with this for us. In the past I've told my daughter that God has control over making babies. That mommies and daddies do their part by being loving and together and when they are ready they ask God for a baby, she's only 6. I told her early last November that we were expecting "Pindot". I wasn't more then 2 or 3 weeks along when the OTC test was negative and my period started. I explained to her that God saw that we weren't really quite ready for "Pindot" to join our family so he took it back for a little while. I explained that God was right, Daddy and I hadn't asked for a #2 baby, "Pindot" was a surprise and we weren't quite ready, God was just showing us that He had someone for us waiting. We talked to her to see if she was comfortable with the idea of a new baby and being a big sister. When we decided to actively TTC, I was pregnant by January.
If yours asks again, keep it simple, keep it honest. Good luck.