You are also correct that there are not “lost souls,” in the sense that there are not souls that are caught between death and eternity unable to “cross over” to “the other side.” After death, a soul is judged and is either saved or damned. A damned soul is in hell; a saved soul is either in heaven or in purgatory.
That said, it may be that purgatory might in some way involve the life one lived here on earth. It is not incompatible with a belief in purgatory to believe that our purgation might include “revisiting” places that were significant during our earthly lives. It may be that occasionally God allows people to see such souls in order to inspire prayers for them or in some way to teach a lesson to the people who see them.
In any case, what you can tell your kids is this: Although Church teaching does not rule out the possibility of ghosts, and although it also does not rule out the possibility that God might allow us to see ghosts on occasion, the Church does forbid us from attempting to initiate occult contact with departed souls.