Answer:
I love the story of the dogwood as well but it probably is not entirely true.
I had the pleasure of being in Israel several years ago and a Jewish historian (a believing Messianic Jew) led us around the country and taught us interesting facts about the Holy land and how it applied to scripture.
Wood was fairly scarce around Jerusalem and most of the trees that did grow were olive trees. In fact many are still around today. Most likely the wood for the cross was fashioned from olive wood since it was in greatest supply.
In fact I took a picture of an olive tree that had cut to show how a cross was made in those days, putting a cross member into a groove cut into the tree. This cross member was probably what Jesus had to carry up Calvary.
If this is used as part of a sermon illustration, I would simply let the children know that many believe this to be true although we have no proof of it in the Bible.
Good question,
Pastor Brad