It's great to use imagination in teaching. It is exciting when a teacher can get the children to picture in their minds what it was like for a certain person in the Bible. However, caution should be taken to differentiate between imagination and what scripture actually tells us.
This can be accomplished with a simple "perhaps" or "The Bible doesn't tell us, but I can just imagine that it happened this way."
Painting a verbal picture to create interest and make your lesson more memorable is not only OK, it is good teaching. Just be sure to distinguish the difference between what God's Word states and what you are imagining. It will show respect to God's Word as it is indeed God's Word.