I think it's always important to be open and honest with your children. I don't think it's appropriate to tell children about terrorism if they don't ask. If they do ask though, I think you should do your best to explain. This should be done according to your child's understanding of other things. Only you know what explanation will be best for your specific child. The explanation could be as simple as, "terrorism is when bad people hurt other people". This isn't the most accurate definition, but it may suffice. It could be a little more elaborate like, "Terrorism is when bad people do bad things to scare other people."